Boris Johnson and Liam Fox are seen as most likely to quit over the customs union issue

Theresa May’s team has privately admitted she may have to accept permanent membership of a European customs union, after a secret wargaming exercise concluded that even Brexiteers such as Michael Gove and David Davis would not resign in protest.

The prime minister has insisted that the UK will leave the common tariff area so it can pursue free trade deals outside the EU. But one of May’s political team told a meeting on March 20 that she and senior aides “will not be crying into our beer” if parliament forces the government’s hand — a position that will enrage some Brexiteers.

The Lords voted last week to stay in a customs union and 10 Tory MPs are expected to do so in the Commons, overturning