Give MPs a free vote on final Brexit deal, John Major says
In a high-profile intervention in the Brexit debate, he warned of a "terrible backlash" from the public if EU withdrawal leaves the UK poorer and weaker, as forecasts suggest.
Sir John called on Mrs May to stand up to the "ultra-Brexit" minority in her party and drop her "red lines" of taking Britain out of the single market and customs union.
The red lines were opposed by a majority in both Houses of Parliament and had "boxed the Government into a corner" in negotiations, making a favourable outcome "impossible", he said.
It was "not credible" to expect to leave the single market, customs union and European Court of Justice while at the same time seeking a-la-carte access to European markets, he said.
He warned: "Unrealistic aspirations are usually followed by retreat. That is a lesson for the negotiations to come.
"They will be the most difficult any Government has faced. Our aims have to be realistic. I am not sure they yet are."
The Conservative Party appeared not to understand business concerns over Brexit, and was only saved from a "haemorrhage of business support" because of fears of Labour's Jeremy Corbyn taking power, said the former PM.
Losing trade advantages in relation to the EU was an act of "economic self-harm" and loss of access to Europe could cost as many as 125,000 jobs in Japanese-owned firms alone.
Speaking in London just two days before Mrs May sets out her own vision for a post-Brexit Britain, Sir John said that the referendum result obliged the Government to negotiate a Brexit deal, but not to pull the UK out of the EU "at any cost".
Mrs May's current position was "tilted to ultra-Brexit opinion", even though hardline leavers had so far been wrong in nearly every promise they made to referendum voters, he said.
The Government's duty was to "negotiate a Brexit, but not any Brexit, not at all costs and certainly not on any terms", said Sir John.
"The true remit can only be to agree a Brexit that honours the promises made in the referendum. But, so far, the promises have not been met and, probably, cannot be met.
"Many electors know they were misled: many more are beginning to realise it. So, the electorate has every right to reconsider their decision."
Mrs May should make clear that the "meaningful vote" promised to MPs on the final Brexit deal will have the options of accepting or rejecting the outcome, sending negotiators back to seek improvements or calling a second referendum, he said.
He called on Mrs May to "let Parliament decide, or put the issue back to the British people".
With only 37% of the electorate actively backing Leave, the referendum did not deliver an "overwhelming mandate to ignore the reservations of 16 million voters who believe it will be a harmful change of direction for our country", said Sir John.
"Of course, the 'will of the people' can't be ignored, but Parliament has a duty also to consider the well-being of the people," he said.
"No-one voted for higher prices and poorer public services, but that is what they may get. The emerging evidence suggests Brexit will hurt most those who have least. Neither Parliament nor Government wish to see that."
Sir John rejected arguments that it was "unpatriotic" not to back Brexit, stating: "It is precisely because I am patriotic that I oppose it.
"I want my country to be influential, not isolated; committed, not cut-off; a leading participant, not a bystander. I want us to be richer, not poorer."