John Major has said the British people have “every right” to reconsider Brexit in a second referendum as he broke his silence on the state of the negotiations with Brussels in a significant intervention.

The former Prime Minister also launched a furious attack on the ultra-Brexiteers in the Conservative Party adding that pre-referendum promises were “wrong“ and described the £350m promise for the NHS after Brexit as a “ridiculous phantom”.

Speaking in central London Sir John, who led the Conservative Party between 1990 and 1997, said Theresa May should offer MPs in the Commons a free vote on the final deal, with the option of putting it to the public in a second referendum.

“Many electors know they were misled,” he said. “Many more are beginning to realise it. So, the electorate has every right to reconsider their decision.”

He later added: “By 2021, after the likely two-year transition, it will be five years since the 2016 referendum. The electorate will have changed. Some voters will have left us. Many new voters will be enfranchised. Others may have changed their mind.

“No-one can truly know what “the will of the people” may then be. So, let Parliament decide. Or put the issue back to the people.”

Sir John called on Ms 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.

Warning that the Government's negotiating position was not realistic, he urged the Prime Minister to be prepared to “change course” and seek a Norway-style solution which would involve accepting single market rules and paying for access to EU markets.

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.

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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.”

In his speech – timed just two days before Ms May delivers her own speech on the future relationship with Brussels – Sir John added that his party appeared to not understand the business concerns over Brexit, and was only saved from a “haemorrhage of business support” because of fears over Jeremy Corbyn entering Downing Street.

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."

Responding to the speech, Tom Brake, the Liberal Democrat Brexit spokesperson, said: “John Major is right, the deal must go back to the people.

“This is the biggest issue facing the nation in over half a century. This deal will define our future. We have one chance to get this right and yet Theresa May is fighting to keep the final say in the hands of her inner circle.”

And Lord Madelson, the Labour peer and supporter of the pro-EU pressure group Open Britain, welcomed the “seismic” speech with “every MP should heed”.

“John Major has stripped away every fig leaf, exposed every lie and revealed the truth about the unmitigated disaster that the Government’s Brexit deal would be for our country,” he added.

But Jacob Rees-Mogg said it was clear the former Prime Minister was attempting to reverse the result of the referendum with “cheap comments and propaganda”.

“We had a democratic vote, the decision has been taken and what he is trying to do is overturn that,” he claimed. “This isn't a statesman-like speech. This is one of somebody grubbing around in the weeds for weak arguments. It is a very poor speech.