Theresa May is facing growing pressure from leading Brexiters to press ahead with fully leaving the customs union or else put future trade deals in jeopardy and risk losing the trust of the British people.

On a trade mission to South America, Boris Johnson warned the prime minister that securing the best trade deals post-Brexit was dependent on leaving the customs union in its entirety with “confidence and brio and zap and dynamism”.

Amid a concerted fightback by Brexiters, Michael Gove, the environment secretary, said that the backstop position, agreed by the cabinet last week in the event that the Irish border issue is not resolved speedily, must only be in place for a “short time”. Tory backbencher Jacob Rees-Mogg, meanwhile, accused the government of weakness for already considering a postponement of its plans to leave the European Union and urged it to show more “backbone” in negotiations.

The MP, who chairs the European Reform Group of pro-Brexit MPs, suggested he was losing faith in the prime minister’s ability to deliver, renewing speculation that he may at some point lead a challenge to her position. “In terms of these negotiations, particularly as they’re led by somebody who backed remain, trust is very important, and it’s very important that the government maintains faith with those who voted to leave,” he said in an interview with the grassroots Tory website Conservative Home.

Rees-Mogg also questioned the prime minister’s commitment to leaving the EU. “I fear we’re getting to the point where you wonder if the government really wants to leave at all,” he said. “I have doubts that what she says is still fine, and what she does is not delivering on what she says and that inevitably raises concerns. She needs to bring the two together.”



Asked about May’s customs partnership model, under which the UK would collect EU import tariffs on behalf of Brussels, he was unequivocally critical: “I’m surprised the prime minister is plodding on with this not very satisfactory idea.”

The foreign secretary also indicated that he felt the clock was ticking on securing a customs deal and avoiding the backstop option, despite earlier this week calling for eurosceptic Tory MPs to give May “time and space” to deliver Brexit. “The prime minister is the custodian of the plan, which is to come out of the customs union, out of the single market and to get on with it, to get on with that project with all convenient speed,” he said in an interview with Bloomberg.

“What people like Argentina, Peru, Chile, outward, free-trading countries, what they want to hear from us is that we are getting on with it with confidence and brio and zap and dynamism.”

Johnson said that Britain should not continue levying the external tariff required by the backstop for very long. “It’s important for people to have a sense of when it’s going to happen and to be able to do it as fast as is reasonably possible,” he added.

He hinted that if the backstop was invoked for any longer than a short period, he would not regard Brexit as having been delivered. “The prime minister has made it absolutely clear that we are coming out of the customs union and we’re coming out of the single market and what is entailed by those promises is very precise,” he said.

A No 10 spokesman said: “The prime minister has made clear we are taking back control of our laws, borders and money, and we are making good progress on that.”

It comes ahead of Jeremy Corbyn’s first visit to Northern Ireland since he became party leader, 20 years since voters approved the Good Friday Agreement. The two-day trip will focus on a return to power-sharing in Stormont and the prevention of a hard border after Brexit, although Corbyn has already come under pressure over his association with the Irish republican movement in the 1980s and 90s.

The Labour leader, who has claimed he played a role in peace-building, has been challenged by the DUP to condemn IRA violence before the visit. He will give a speech at Queen’s University Belfast, where the IRA murdered 29-year-old law lecturer Edgar Graham in 1983, and visit the Irish border.

A spokesman said: “Jeremy is looking forward to discussing the vital issues of peace, democracy and trade on this visit. We must not allow the current stalemate in Stormont or the Tories’ chaotic handling of Brexit to undermine the hard-won peace, which both communities struggled for.”