The campaign is credited with having had a key role in bringing about the United Kingdom European Union membership referendum, held in June 2016. Prominent Conservative MEP Daniel Hannan wrote in his 2016 book "What Next?" that "The People's Pledge was the most effective popular campaign I have ever been involved with, and must be reckoned one of the most successful pressure groups in British history." James Forsyth, writing in "The Spectator," said that the campaign's recruitment of Boris Johnson as a supporter had made an EU referendum almost certain: "Boris Johnson's decision while campaigning last month to sign up to the campaign for an in/out referendum on EU membership could be a key moment in the history of the Conservative party – the moment when the party's balance of power tipped decisively in favour of a referendum." Along with Forsyth, Paul Goodman of "ConservativeHome" and Leo McKinstry of the "Daily Express" have cited the success of the campaign as a main reason why a referendum became a possibility.