In London and Brussels, in talks that could determine Britain’s future, negotiators are homing in on a common goal: how to rein in the actions of a future pro-hard Brexit British government.

The focus in discussions between the UK’s Conservative and Labour parties is on providing assurances that a new Tory prime minister does not rip up any cross-party accord on future relations with the EU.

Diplomats in Brussels are concerned with a similar issue, as the EU’s 27 other member states consider Britain’s request to delay its departure from the bloc. A big preoccupation ahead of a crucial summit on Wednesday is how to prevent a more Eurosceptic UK government from disrupting the bloc’s affairs from within.

Both sets of concerns are personified by one politician in particular: Boris Johnson, the former UK foreign secretary who led the triumphant Leave campaign in the 2016 EU referendum and who hopes to succeed Theresa May as prime minister in the near future.

Labour is worried that a prime minister Johnson could discard any agreement by Mrs May that commits the UK to closer post-Brexit relations with the EU than the UK government currently seeks.

Other EU governments — notably France — fret that if the UK is granted a lengthy delay to its Brexit date, the country could wreak havoc with decisions in the European Commission, the European Council of member states and the European Parliament, particularly if a full-blooded Eurosceptic is in Downing Street.

The idea of a ‘Boris lock’ is ridiculous. Parliament can’t bind its successors, no matter what the prime minister might agree with Labour or the EU

But the problem for the Labour-Conservative talks, perhaps also for the deliberations in Brussels, is that restricting the conduct of a future British government is far more easily said than done, particularly if the UK decides to go down a more antagonistic path.

“The idea of a ‘Boris lock’ is ridiculous,” said a senior Conservative MP. “Parliament can’t bind its successors, no matter what the prime minister might agree with Labour or the EU.”

Labour remains agitated about Mr Johnson as Westminster is absorbed by speculation that Mrs May’s last days as prime minister are approaching.

Although Mrs May has said she would only resign once her Brexit deal is passed by parliament, most Conservative MPs believe she will leave office in the autumn. Others believe she will have been pushed out by the summer

Mr Johnson is the favoured candidate of the party’s grassroots, according to surveys by the ConservativeHome website.

He is also the favourite in the betting markets — followed by former Brexit secretary Dominic Raab and environment secretary Michael Gove.

All three are strong Brexit proponents, and Mr Johnson and Mr Raab are fierce critics of Mrs May’s exit deal with the EU, although they voted for it in the House of Commons at the third time of asking. Hence Labour’s fear that, without strong guarantees, any deal with Mrs May might fail to last out the year.

While Labour’s negotiating team acknowledges that a future parliament could renegotiate any agreement, it wishes to ensure that the next Conservative prime minister cannot change the deal before an election.

Rebecca Long Bailey, Labour’s spokesperson on business, told the BBC at the weekend that any deal with the Conservatives must be “entrenched so that a future Conservative leader wouldn’t be able to rip up the changes that have been agreed” — in other words, “Boris-proofed”.

John McDonnell, Labour’s shadow chancellor, said on Tuesday that any protections to stop a deal being unpicked also had to be in a treaty. “It’s more than it being in legislation, it’s about the agreement we have with the EU,” he said.

Meanwhile there is deep disdain for Mr Johnson among many European governments, which see him as the wayward leader of a Brexit campaign built on false promises.

But that is offset by growing impatience with Mrs May’s government, which lacks the authority in Westminster to see through on agreements made in Brussels. “Give us anyone who has a majority,” said one senior EU diplomat, who hoped for a quick resolution to the Brexit saga, one way or another.

The EU has moved to shield itself against a change of guard in London by making clear that the withdrawal agreement negotiated with Mrs May is now in effect untouchable, regardless of who is in Downing Street.

Recommended A long Brexit pause makes sense for the EU and the UK

Fear over a “rogue” Brexiter government subverting EU business has played a big role in raising concerns about the costs to the EU of approving a long delay to Britain’s departure date.

Eurosceptic MPs have already urged the UK to act as a wrecker from within, especially if restrictions are attached to a Brexit delay.

At a meeting of Europe ministers on Tuesday, Greece noted that, while no deal might be damaging, it might be no worse than “being held hostage” to a war within the Tory party while Britain remained a member state.

One EU diplomat suggested that the terms of an extension also needed to be “Bojo proof” — to allow the bloc to cut short UK membership if Mr Johnson “or anyone irresponsible is prime minister one day and threatens to wreak havoc within the EU”.

Senior French officials have privately suggested review clauses — potentially at intervals of two or three months.

A spokesman for Mr Johnson declined to comment on “speculation about the future”.

But Jacob Rees-Mogg, head of the pro-Brexit European Research Group of Conservative MPs, who has been associated with Mr Johnson’s nascent leadership campaign, told the Financial Times that such restrictions would likely fail.

“To limit the power of a member state would require treaty change,” he said. “The British government cannot give the EU a nod and a wink to promise good behaviour . . . If we are stuck in we must use the remaining powers we have to be difficult.”

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