TORY infighting over Brexit intensified yesterday, with Boris Johnson demanding the Prime Minister crash the UK out of the customs union.

The Tory Foreign Secretary used a newspaper column to very publicly tell Theresa May to make a clean break from the EU.

It’s the second time in a week that Johnson has expressed fears about his leader’s enthusiasm for Brexit.

Speaking to the press during a trip to Latin America, he said the Prime Minister needed to commit to her promises and not “betray” Brexiteers by keeping the UK in the customs union.

In his column yesterday, Johnson called on the government to make the UK ”truly global again.”

He said: “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”.

There are two customs union options being discussed in cabinet.

The first, the customs partnership, would see Britain stick closely to the EU’s customs regime to remove the need for new checks at the border. The UK would collect tariffs set by the EU on behalf of the EU.

If those goods didn’t leave the UK and UK tariffs on them were lower, companies could then claim back the difference.

Brexiteers hate that proposal, believing it leaves the UK at the mercy of EU laws – influential backbench MP Jacob Rees-Mogg called it as “completely cretinous”.

The second option, maximum facilitation, known as Max Fac, would use as yet un-invented technologies and “trusted trader” schemes to allow companies to pay duties in bulk every few months rather than every time their goods cross a border.

There is little detail on his this would work.

Over the weekend, Rees-Mogg urged the Prime Minister to be “stronger” in her dealings with the EU.

He told her not to pay the £50 billion Brexit divorce bill unless a new trade agreement is reached.

Rees-Mogg, who heads up a powerful committee of backbench pro-Brexit MPs and who has been tipped as a future leader of the party, denied he would challenge May for her job.

He warned her that she had made a “mistake” in her negotiations on the customs union.

He told the BBC’s Andrew Marr show: “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.”

Meanwhile, deputy leader of the Labour party, Tom Watson, said no decision had been made on whether his MPs would be whipped to oppose membership of the single market in the vote next month.