Boris Johnson has told Theresa May the UK must come “fully out” of the EU customs union, amid growing concern among Brexiteers about the UK’s future relationship with the trading bloc.

The foreign secretary said his recent trip to South America highlighted the potential for new trade deals with countries that are “realms of gold”, but warned there was “absolute unanimity” that Britain would be a “valid trading partner” only if it left the customs union.

It comes as Eurosceptic MPs warned the prime minister over the cabinet’s decision to agree a “backstop” option that would see the UK retain the same tariffs as the customs union for a limited time if no other deal on the Northern Irish border is agreed.

Leading Brexiteer Jacob Rees-Mogg called the potential solution “a real problem” and urged Ms May to take a tougher stance in negotiations with Brussels.

Adding to pressure on the prime minister, Mr Johnson said new trade deals were dependent on the UK coming “fully out” of the customs union.

Writing in The Daily Telegraph, Mr Johnson said: “Now is our moment not to be less European – we can do a great free trade deal with the EU that will benefit both sides – but to be truly global again.

“Now is the time to create deals with these dynamic countries. (Chile alone has 24 such deals with 64 nations comprising 85 per cent of global GDP). But our Latin American partners are emphatic: if this is to work, we must come fully out of the EU customs union.”

Referring to his trip to South America, he said: “There is absolute unanimity that if we are to be a valid trading partner, then we must take back control – as the PM has said – of our tariff schedules, and do deals that are unhindered and uncomplicated.

“For Latin America and for Britain there are realms of gold on either side of the Atlantic. All we need now is to rediscover that global spirit.”

Mr Johnson claimed he was the first British foreign secretary in 50 years to visit Peru and the first in 25 years to go to Chile and Argentina, suggesting this was because being a member of the EU had made the UK “more Eurocentric and less instinctively global than we had been”.

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The foreign secretary caused a stir during his recent trip by saying the government must “get on with it” and take Britain out of the customs union.

He said: “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, and that is what we are going to do.

“I think 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.”

His latest warning comes after Mr Rees-Mogg told Ms May she was making “an error” by not taking a tougher line against the EU in negotiations about the Northern Irish border.

The North East Somerset MP, who chairs the European Research Group of Eurosceptic Tories, had urged the prime minister to “call their bluff” during talks with Brussels by suggesting the only way there would be a hard border after Brexit is if the EU creates one.

He told the BBC: “The prime minister said in her Mansion House speech that she wasn’t going to do this, I think that is a mistake.

“I think it is the obvious negotiating position to have. Bearing in mind the Irish economy is heavily dependent on its trade with the United Kingdom, it is overwhelmingly in the interests of the Republic of Ireland to maintain an open border with the United Kingdom.

“I think, if you are going into a negotiation, you should use your strongest cards and just to tear one of them up and set hares running on other issues is, I think, an error.”

He described the “backstop” option as a “real problem”, saying: “The customs union on its own does not solve the seamless border issues ... you require regulatory alignment as well, and that means the single market.

“If we were to stay as a rule-taker, as a vassal state, for an indeterminate period, I don’t think that would be delivering on Brexit. And if you offer a backstop that is more attractive than anything you’re likely to negotiate, from the other side’s point of view, the backstop ends up becoming a frontstop.”

Jeremy Corbyn on single market membership after Brexit

Ms May faces a battle to hold her party together as she prepares for crucial votes on Brexit in the House of Commons.

The government hopes to overturn all 15 amendments that the House of Lords made to the flagship EU Withdrawal Bill, including one that would force ministers to negotiate a new customs union with the EU.

There are believed to be enough Tory rebel MPs to uphold the amendment – a change that would be seen as completely unacceptable by Eurosceptics and could drive Tory divisions wide open.

There could also be a Commons majority in favour of keeping a Lords amendment that would allow Britain to retain access to the EU single market by joining the European Economic Area (EEA), should Labour agree to back it.

Jeremy Corbyn has previously dismissed the Norway-style EEA option, but his deputy, Tom Watson, said Labour had not decided how to vote on the issue.

Mr Watson told ITV: “We haven’t decided our whipping arrangements yet.