If Britain votes to leave the EU "on the basis of half-truths and misunderstandings the gravediggers of our prosperity will have some serious questions to answer", Sir John Major has said.
The former prime minister, speaking alongside Prime Minister David Cameron and Labour's Harriet Harman, said Britain votes out the country will be "diminished as an influence in the world" and a "truly great Britain will have shrunk down to a little England".
He said remaining in the EU means Britain can have more of an influence in times of international crisis - citing the EU sanctions on Russia and the EU nuclear deal with Iran.
"We simply could not have done that on our own," he said.
He also warned it could put the UK in jeopardy as Scotland may seek to leave the union with Britain, Wales may be "grumpy" and Northern Ireland divided by border controls at the south at the edge of the EU.
Former Prime Minister Sir John Major has said the decision to withhold some of the correspondence about the Iraq war will be "very embarrassing" for Tony Blair.
Sir John Major told Radio 4's Today programme: “I think it is a pity the papers are going to be withheld for several reasons. Firstly, they will leave suspicions unresolved and those suspicions will fester and maybe worsen."
He also argued that the ruling ran counter to Mr Blair's actions to make government more transparent.
"And secondly, in many ways I think withholding them is going to be very embarrassing for Mr Blair, not least of course because he brought in the Freedom of Information Act into law when he was in government."
Former Prime Minister Sir John Major has paid tribute to BBC broadcaster John Cole, saying that when he "spoke, everyone listened."
Sir John Major said he was "too sensitive" over press coverage towards him when he was Prime Minister, as he gave evidence at the Leveson Inquiry.
The former Prime Minister - who held office from 1990 to 1997 - said his poor relationship with the press was down to his decision not to court them which, he added, would have been "undignified".
Sir John added that the press cannot hold the Government to account if there is too much "chumminess" between them.
Sir John Major has been speaking to ITV about serving as one of the Queen's 12 Prime Ministers. He is also patron of the Diamond Jubilee Trust.
He will be among those attending the Thanksgiving Service at St Paul's Cathedral later this morning.