Lancaster has stated his disagreement with the UK Government's policy on the 2003 invasion of Iraq. In an interview with the BBC, he stated "It may well be much harder to get the British public to back other overseas adventures by the military because of what's happened in Iraq". Lancaster voted against legislation allowing gay couples to marry at the Bill's second reading, but supported minor 'tidying up' legislation supporting the principle once the main Bill had passed through the House of Commons. He later tweeted in July 2016 that he had been wrong to have opposed the second reading.