Question: What position did the prime minister hold when UK joined the common market?

full_context: Document 1: 1975 United Kingdom European Communities membership referendum: The United Kingdom European Communities referendum of 1975, also known as the Common Market referendum and EEC membership referendum was non-binding referendum held on 5 June 1975 in the United Kingdom to gauge support for the country's continued membership of the European Communities (EC) -- often known at the time as the ``Common Market ''-- which it had entered on 1 January 1973 under the Conservative government of Edward Heath. Labour's manifesto for the October 1974 general election had promised that the people would decide`` through the ballot box'' whether to remain in the EC.
Document 2: Elizabeth II: The absence of a formal mechanism within the Conservative Party for choosing a leader meant that, following Eden's resignation, it fell to the Queen to decide whom to commission to form a government. Eden recommended that she consult Lord Salisbury, the Lord President of the Council. Lord Salisbury and Lord Kilmuir, the Lord Chancellor, consulted the British Cabinet, Winston Churchill, and the Chairman of the backbench 1922 Committee, resulting in the Queen appointing their recommended candidate: Harold Macmillan.
Document 3: Elizabeth II: In 1956, the British and French prime ministers, Sir Anthony Eden and Guy Mollet, discussed the possibility of France joining the Commonwealth. The proposal was never accepted and the following year France signed the Treaty of Rome, which established the European Economic Community, the precursor to the European Union. In November 1956, Britain and France invaded Egypt in an ultimately unsuccessful attempt to capture the Suez Canal. Lord Mountbatten claimed the Queen was opposed to the invasion, though Eden denied it. Eden resigned two months later.
Document 4: The Sun (United Kingdom): Politically, The Sun in the early Murdoch years, remained nominally Labour. It supported the Labour Party led by Harold Wilson in the 1970 General Election, with the headline "Why It Must Be Labour" but by February 1974 it was calling for a vote for the Conservative Party led by Edward Heath while suggesting that it might support a Labour Party led by James Callaghan or Roy Jenkins. In the October election an editorial asserted: "ALL our instincts are left rather than right and we would vote for any able politician who would describe himself as a Social Democrat."
Document 5: Dudley Smith: Smith served as a junior minister for the British Army and Employment during the Edward Heath government of 1970 to 1974.
Document 6: Government of the United Kingdom: The current prime minister is Theresa May, who took office on 13 July 2016. She is the leader of the Conservative Party, which won a majority of seats in the House of Commons in the general election on 7 May 2015, when David Cameron was the party leader; although at the last general election she failed to secure a majority government. Prior to this, Cameron and the Conservatives led a coalition from 2010 to 2015 with the Liberal Democrats, in which Cameron was prime minister.
Document 7: Brexit: In a referendum on 23 June 2016, 51.9% of the participating UK electorate voted to leave the EU, out of a turnout of 72.2%. On 29 March 2017, the UK government invoked Article 50 of the Treaty on the European Union. The UK is thus due to leave the EU at midnight on 30 March 2019 Central European Time (11 pm on 29 March 2019 GMT).
Document 8: European Union law: Following the election of the UK Labour Party to government in 1997, the UK formally subscribed to the Agreement on Social Policy, which allowed it to be included with minor amendments as the Social Chapter of the 1997 Treaty of Amsterdam. The UK subsequently adopted the main legislation previously agreed under the Agreement on Social Policy, the 1994 Works Council Directive, which required workforce consultation in businesses, and the 1996 Parental Leave Directive. In the 10 years following the 1997 Treaty of Amsterdam and adoption of the Social Chapter the European Union has undertaken policy initiatives in various social policy areas, including labour and industry relations, equal opportunity, health and safety, public health, protection of children, the disabled and elderly, poverty, migrant workers, education, training and youth.
Document 9: Heath ministry: Edward Heath of the Conservative Party formed the Heath ministry and was appointed Prime Minister of the United Kingdom by Queen Elizabeth II on 19 June 1970, following the 18 June general election. Heath's ministry ended after the February 1974 general election, which produced a hung parliament, leading to the formation of a minority government by Harold Wilson of the Labour Party.
Document 10: 2016 United Kingdom European Union membership referendum: The planned referendum was included in the Queen's Speech on 27 May 2015. It was suggested at the time that Cameron was planning to hold the referendum in October 2016, but the European Union Referendum Act 2015, which authorised it, went before the House of Commons the following day, just three weeks after the election. On the bill's second reading on 9 June, members of the House of Commons voted by 544 to 53 in favour, endorsing the principle of holding a referendum, with only the Scottish National Party voting against. In contrast to the Labour Party's position prior to the 2015 general election under Miliband, acting Labour leader Harriet Harman committed her party to supporting plans for an EU referendum by 2017.To enable the referendum to take place, the European Union Referendum Act was passed by the Parliament of the United Kingdom. It extended to include and take legislative effect in Gibraltar, and received royal assent on 17 December 2015. The Act was, in turn, confirmed, enacted and implemented in Gibraltar by the European Union (Referendum) Act 2016 (Gibraltar), which was passed by the Gibraltar Parliament and entered into law upon receiving the assent of the Governor of Gibraltar on 28 January 2016.
Document 11: Brown ministry: Gordon Brown formed the Brown ministry after being invited by Queen Elizabeth II to begin a new government following the resignation of the previous Prime Minister of the United Kingdom, Tony Blair, on 27 June 2007. He took office as Prime Minister, a title he would hold until his resignation on 11 May 2010. In his inaugural cabinet, Brown appointed the United Kingdom's first female Home Secretary, Jacqui Smith.
Document 12: Blair ministry: Tony Blair originally formed the Blair ministry in May 1997 after being invited by Queen Elizabeth II to form a new government following the resignation of the previous Prime Minister, John Major of the Conservative Party, as a result of the Labour Party's landslide victory at the 1997 general election. He would serve as the Prime Minister for three successive ministries and parliamentary terms until his resignation on 27 June 2007. His Cabinet was reshuffled for each new parliament along with a few minor changes during each term.
Document 13: Prime Minister of the United Kingdom: The Prime Minister of the United Kingdom is the head of Her Majesty's Government in the United Kingdom. The Prime Minister (informally abbreviated to PM) and Cabinet (consisting of all the most senior ministers, most of whom are government department heads) are collectively accountable for their policies and actions to the Monarch, to Parliament, to their political party and ultimately to the electorate. The office is one of the Great Offices of State. The current holder of the office, Theresa May, leader of the Conservative Party, was appointed by the Queen on 13 July 2016.
Document 14: 1975 United Kingdom European Communities membership referendum: The United Kingdom European Communities membership referendum, also known as the Referendum on the European Community (Common Market), the Common Market referendum and EEC membership referendum took place on 5 June 1975 in the United Kingdom to gauge support for the country's continued membership of the European Communities (EC) -- often known at the time as the ``European Community ''and the`` Common Market'' which it had entered on 1 January 1973 under the Conservative government of Edward Heath under the provisions of the Referendum Act 1975. Labour's manifesto for the October 1974 general election had promised that the people would decide ``through the ballot box ''whether to remain in the EC.
Document 15: Brexit: In a referendum on 23 June 2016, 51.9% of the participating UK electorate voted to leave the EU, out of a turnout of 72.2%. On 29 March 2017, the UK government invoked Article 50 of the Treaty on the European Union. The UK is thus due to leave the EU on 29 March 2019.
ground_truth: Prime Minister of the United Kingdom
sub_queries: who was prime minister when we joined the common market Answer: Edward Heath
Edward Heath >> position held Answer: Prime Minister of the United Kingdom