In an extraordinary intervention, Sir John was to cast doubt on the “enticing” promises being offered to voters and attack Ms May and her ministers for saying nothing about massive Brexit costs – including a potential £60bn divorce bill – and major potential damage to the NHS and welfare.
As Lords debated the Bill that will allow Ms May to trigger Brexit nearby, Sir John planned to reproach the Prime Minister for trying to use Parliament as a “rubber stamp” for her plans.
Echoing a recent speech by Tony Blair, he would demand those concerned about Brexit be allowed to speak up, denying they are ‘opposing the will of the people’, and instead stating “they are the people”.
The comprehensive take-down of the Government’s approach – revealed in the text of his speech released shortly before he took to the stage – also saw Sir John warn Ms May of building close ties with Donald Trump and of already souring the atmosphere on the continent with “cheap rhetoric”.
Sir John’s intervention is likely to anger Brexiteer Tories already aggravated by Michael Heseltine’s pledge to defy Ms May over her drive to quit the EU.
Targeting the false optimism of ministers, the ex-PM wrote in his speech: “The hopes of those who favoured leaving the European Union are sky-high. We are told that countries ‘are queuing up to do trade deals with us’. That ‘our best days lie ahead’.
“It all sounds very enticing. And, for the sake of our country, I hope the optimists are proved right. But I’m not sure they will be. My own experience of international negotiations, and the national self-interest that accompanies them, makes me doubt the rosy confidence being offered to the British people.”
He pointed out that speeches coming from Brexiteer ministers talk about what the UK will take out of an EU deal, before adding: “We hear nothing about what our country may have to give in return.
“If anyone genuinely believes that Europe will concede all we wish for and exact no price for doing so, then they are extraordinarily naive.”
Sir John then turned his guns directly at Ms May’s Brexit White Paper for making no mention of the divorce bill for Britain – potentially up to £60bn –which EU chief negotiator Michel Barnier is pursuing.
While Sir John said he finds the figure contentious, he went on: “The bill will be substantial. Billions, not millions, and very unpalatable. It will come as a nasty shock to voters who were not forewarned of this – even in the recent White Paper.”
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He added: “When you leave any club, you are obliged to settle your debts, and that is what the European Union is going to expect the UK to do.”
Sir John then raised questions over the Prime Minister’s stated goal of signing both a divorce agreement and a new free trade deal by spring 2019 – something also doubted by some Tory MPs and European politicians – due to the complex nature of what must be agreed.
Ms May has said if she does not get the deal she wants, she will be prepared to quit the EU with no deal, revert to trading with the bloc on WTO rules, and transform the UK in to an enterprise economy.
But Sir John said she and her ministers have failed to explain what this would mean, branding it “the worst possible outcome” with “worrying implications for public services such as the NHS”.
He said: “We cannot move to a radical enterprise economy without moving away from a welfare state.
“Such a direction of policy, once understood by the public, would never command support. It would make all previous rows over social policy seem a minor distraction.”
Sir John also dealt with Ms May’s approach to the Trump administration, reminding the Prime Minister that the UK is the junior partner in the ‘Special Relationship’.
He said: “If we disagree with American policy, we may weaken our ties. But if we support it slavishly, we become seen as an American echo, an invidious role for a nation that has broken free from Europe to become more independent.
“And inevitably there will be disagreements, the US wish to contain China and engage Russia, we wish to contain Russia and engage China.
“We seem likely to disagree also on refugees, free trade, the legality of Jewish settlements, and climate change. How many disagreements can there be before even the closest of ties begin to fray?”
The former prime minister said UK negotiators will need to show the highest statesmanship to secure any sort of deal but instead lamented that, “behind the diplomatic civilities, the atmosphere is already sour”.
He went on: “A little more charm, and a lot less cheap rhetoric, would do much to protect the UK’s interests.”
Tory Brexiteer MPs called for Lord Heseltine to be sacked from his post advising government after he said he would vote to amend Ms May’s Article 50 Bill on Sunday.
Meanwhile, Tony Blair gave a speech in London recently calling for people to “rise up” against Brexit, while Lord Mandelson wrote in The Independent demanding the British people have the final say on Ms May’s deal, possibly in a second referendum.
In response to Sir John’s speech, a No 10 source said: “The Government is determined to make a success of our departure from the European Union and to move beyond the language of leave and remain to unite our country.
“The Prime Minister set out her twelve negotiating objectives for Brexit in January. We have a clear plan to get the best deal for the United Kingdom and are going to get on with the job of delivering it.”
Labour’s Chuka Umunna, a supporter of Open Britain, the campaign group calling for close links to be retained with Brussels, said: “It is absolutely right to warn that if the Government continues on its current course, we will end up with the kind of hard, destructive Brexit that costs jobs and damages our economy.
“The Government have promised to get a deal that delivers the ‘exact same benefits’ as we have today. Sir John is right to question how likely that is, but there is no chance at all of it being delivered if our European partners across the negotiating table continue to be demonised.”
Liberal Democrat Leader Tim Farron said: “The Conservatives should listen to the likes of John Major and Heseltine instead of sneering at them. These are people with huge experience of negotiating with Europe, while the Brexiteers have no clear strategy.
“There is nothing patriotic about shutting down debate on the risks of a hard Brexit for our country’s prosperity and security.”