Former rivals John Major and Tony Blair both appeared at an election meeting organized by the organization "Final say", which wants a second referendum on Brexit to be held before a possible exit from the EU.

John Major, Conservative Prime Minister 1990-97, called Brexit "the worst foreign policy decision he experienced" and warned that an EU exit would make the country poorer and perhaps lead both Scotland and Northern Ireland to leave the Union.

John Major was Prime Minister and Party Leader of the Conservative Party 1990-97. Here he appears during a speech at the Konrad Adenauer Foundation in Berlin 2014. Photo: Michael Sohn / TT-AP

- When the country voted for Brexit, it was against the backdrop of fiction and unenforceable promises. Not allowing a new referendum is unprecedented and frankly undemocratic, Major said in a video message to meeting participants in London.

Blair: Brexit a delusion

Directly after Major spoke Tony Blair, who took over the Prime Minister post from Major after a major victory for the Labor Party in the 1997 election.

Blair began by jokingly thanking God for his former opponent, and then gave a lengthy speech in which he said it was "undemocratic" to include Brexit in a parliamentary election.

"Brexit is a comforting delusion, a substitute for the uncomfortable challenges in the form of a changing world," he said according to the BBC.

Focus on uncertain constituencies

Implementing Brexit is the prime promise made by Prime Minister Boris Johnson and his party ahead of Thursday's election. "Final say" campaigns are therefore in 25 constituencies where the outcome is uncertain, in the hope that it will lead to no party gaining a majority.

The idea is that the opposition parties should then be able to ensure that a second referendum is made, possibly with the support of members who have been excluded from the Conservative Party in order to remain in the EU.