Boris Johnson has stressed the need for the UK to come “fully out” of the EU customs union if the UK is to be a global trading nation.

The Foreign Secretary’s latest intervention comes after influential backbench Tory Brexiteer Jacob Rees-Mogg called on Theresa May to take a tougher line with Brussels in the negotiations.

Mr Johnson said that his recent trip to Latin America – during which he urged the Prime Minister to “get on with it” and take Britain out of the customs union “as fast as is reasonably possible” – made it clear that potential trading partners wanted the UK out of the EU tariffs arrangement.

Boris Johnson in Chile during his Latin American tour where he says it was made clear by potential trade partners the UK must fully leave the customs union (Stefan Rousseau/PA)
Boris Johnson in Chile during his Latin American tour where he says it was made clear by potential trade partners the UK must fully leave the customs union (Stefan Rousseau/PA)

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.”

He said that it was time to create deals with the “dynamic countries” he had visited “but our Latin American partners are emphatic: if this is to work, we must come fully out of the EU customs union”.

If the UK is 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”.

Brexiteers have been alarmed by delays in Cabinet decision-making over the future customs model and the prospect of a backstop deal which could see the UK closely aligned to EU rules for a number of years if alternative ways of resolving the Irish border issue are not found.

Mr Rees-Mogg, chairman of the European Research Group of Tories, claimed the Government had proposed “over-complicated” solutions to the customs problem and must be prepared to tell Brussels it will walk away without paying the almost £40 billion Brexit divorce bill – potentially leaving the bloc in the red.

On BBC1’s Andrew Marr Show, Mr Rees-Mogg said Mrs May had made a mistake over her approach to the Irish border issue, one of the most contentious aspects of the negotiations, by ruling out the prospect of unilaterally keeping an open frontier after Brexit.

Mr Rees-Mogg said: “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. Bear 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 said the Government’s plans for a “backstop” was “a real problem” and could leave the UK a “vassal state for an indeterminate period”.

Mr Rees-Mogg added: “Basically, the deal is very simple – we are paying a very large amount of money, £40 billion, and in return we want a trade deal.

“Everything else is essentially incidental to that.”

The UK should make clear to the EU that “if we don’t get the trade deal we want, you don’t get the money”.

Meanwhile deputy Labour leader Tom Watson said no decision had yet been made on whether the party’s MPs would be whipped to oppose membership of the single market when the Commons votes on the issue in June.

“We haven’t decided our whipping arrangements yet,” he said on ITV’s Peston on Sunday, adding: “We haven’t made a decision yet, but we’ve been pretty clear that there are deficiencies in the Norway model that might not work for a bespoke UK deal.”