The diplomatic row that led to Britain's ambassador to the United States stepping down has taken a new turn after it emerged the journalist who allegedly leaked confidential messages has close links to the Brexit Party.

Political journalist Isabel Oakeshott, the reputed leaker, is in a relationship with party chairman and European Parliament member Richard Tice.

In the messages, ambassador Kim Darroch called US President Donald Trump "clumsy and inept" and days later, he was forced to step down after Trump said he would have nothing to do with him.

As soon as the story emerged, leading Brexit supporters, including Nigel Farage and Tice, called for Darroch to be replaced with a "pro-Brexit businessman", because so much of the argument for Brexit has been made based on obtaining a beneficial trade deal between the United Kingdom and the US.

A high-ranking British diplomat told The Sunday Times newspaper, "It feels like there are a lot of Brexit Party-Faragist fingerprints around this," and Tice did not deny that he and Oakeshott had discussed the cables before publication, saying, "I'm not going to get into details about a personal, private conversation. I knew about it at the same time as the Foreign Office."

The Metropolitan Police has opened a criminal investigation into the leak, while computer hacking by a foreign state has been ruled out.

Assistant Commissioner of the Met Police Neil Basu said he was "satisfied there has been damage caused to UK international relations and there would be clear public interest in bringing the person or people responsible to justice", but his warning that the publication of any further leaks could lead to arrest was criticized as a threat to press freedom. Police insiders said it was meant to let the mole know they were on his or her trail.

Tice denied he had any involvement in what has happened and said he is not looking to replace Darroch. With current British Prime Minister Theresa May entering her final days in office after three years of failure to make progress over Brexit, her successor as Conservative Party and national leader, Boris Johnson or Jeremy Hunt, will have to fill the ambassadorial vacancy.

In a live televised debate last week before the resignation, Hunt said that should he become leader, Darroch would continue in his position until his retirement. Johnson did not offer such support and it has been reported that this was what prompted Darroch to quit, a decision Hunt called "a black day for British diplomacy".

Oakeshott is an award-winning, former political journalist at The Sunday Times and was the ghost writer of a book called The Bad Boys of Brexit, about what went on behind the scenes at the Leave.

Eu campaign group during the 2016 referendum campaign. Farage and Tice are two of the self-proclaimed "bad boys" and were among the first Britons to meet Trump following his election victory.