Brexiteers like Boris Johnson are prepared to sacrifice the Good Friday Agreement for their project

In a leaked letter to Theresa May, Boris Johnson said that ‘it is wrong to see the task as maintaining ‘no border’ on the island of Ireland after Brexit’

No instalment of the Brexit saga would be complete without a foot-in-mouth appearance by Boris Johnson (BBC)

Once again, the concessions Theresa May makes to hardline Tory Brexiteers come back to haunt her. Earlier this month Downing Street ruled out “a customs union” with the EU, while under pressure from the Brextremists issuing their usual threats to topple May.

The Prime Minister needlessly closed off her options in order to survive another week in office. The pattern has been repeated in other areas. On her recent trip to China, May threw the ever-ravenous Eurosceptics another sweetie by promising to end free movement for EU citizens during the two-year transitional phase from March next year. Ministers believe she will have to back down, and allow EU citizens who come to the UK during this period to remain permanently, in order to secure a transitional deal vital for British business at next month’s EU summit.

Similarly, the Prime Minister sticks to her rhetoric about ending the European Court of Justice’s role in the UK, while quietly preparing to allow its writ to run in some areas. If and when a deal is done, May’s red lines will begin to look what the arch-Brexiteer Jacob Rees-Mogg would call “rather pink”.

May’s sweetener to his anti-customs union gang contributed to today’s row with the EU over Northern Ireland. A continuing customs agreement would not entirely solve the thorny border question, but would make a solution much easier. Today’s draft EU legal text on the withdrawal agreement – proposing a “common regulatory area” between the EU and Northern Ireland in the event of no UK-EU trade deal – was attacked by May’s aides as “cherry-picking” the phase-one agreement struck in December. But it was May who agreed then to “full regulatory alignment” on the island of Ireland as a fallback option if there were no deal.

The decision by Michel Barnier, the EU’s chief negotiator, to highlight this option was seen in London as unnecessarily provocative. May said at Prime Minister’s Questions today that no prime minister could allow a border between the UK and EU in the Irish Sea, in effect cutting Northern Ireland off from the rest of the UK. Certainly no prime minister who depends on the votes of 10 Democratic Unionist Party MPs at Westminster, she might have added.

The UK pot is calling the EU kettle black. The compromise agreed by the Cabinet’s Brexit subcommittee at Chequers last week was a masterclass in “cherry-picking”: the UK will demand the freedom to diverge from EU regulations in some areas, while sticking to them in others. This ignores repeated warnings that the EU27 will never allow such a “have cake and eat it” approach. “The UK is still in La La Land,” said one EU official.

No instalment of the Brexit saga would be complete without a foot-in-mouth appearance by Boris Johnson and this time he managed a double dose. His likening of the Irish border to London’s congestion charge was intended to talk up technological solutions but was insensitive. It was compounded by the leak of his letter to May, saying that “it is wrong to see the task as maintaining ‘no border’ on the island of Ireland after Brexit”, while urging her to prevent the border becoming “significantly harder”. Allies insist the Foreign Secretary’s letter has been selectively quoted. But it will fuel fears that Brexiteers are so obsessed with their project that they are prepared to sacrifice the Good Friday Agreement. Owen Paterson, the former Northern Ireland secretary, has suggested the agreement has “outlived its use”.

Despite today’s war of words between London and Brussels, a way may yet be found to kick the Northern Ireland can down the road again so that trade talks can start next month. May could help by addressing customs arrangements in her major Brexit speech on Friday.

As well as lowering the temperature on Northern Ireland, committing to some form of customs union could head off the real threat of a humiliating Commons defeat on the issue at the hands of Tory pro-Europeans and opposition parties. That would deny Jeremy Corbyn a major coup, after he came out in support of a customs union on Monday. For good measure, it would reassure business leaders, who at present see Corbyn’s Labour as more pro-business than the Tories on Brexit.

A customs union would not be such a big leap for May. In her Lancaster House speech in January last year, she said she wanted “a customs agreement” with the EU, and had an open mind about a new deal, being an associate member of the existing EU customs union or a signatory to some elements of it.

Of course, the 60 hardline Tory Brexiteers would cry foul, and renew their threats to oust May. They always do. After Corbyn’s deft move, there is a Commons majority for a customs union. Even some cabinet ministers hope privately that it will assert itself, and force May to accept such an arrangement. It would be better for May to get ahead of the game than suffer a Commons defeat. At some point, she will have to recognise the parliamentary arithmetic, and stop allowing the Brexiteers’ tail to wag the Government dog.