The police have been drawn into claims of illegitimate foreign donations to Ukip linked to the pro-Donald Trump Breitbart website, in the run up to the Brexit referendum

A Labour MP has written to the Metropolitan Commissioner, Cressida Dick, and to the Electoral Commission watchdog, to find out if the allegations have been fully investigated.

The move follows revelations from two Ukip whistleblowers that they filed complaints to the Commission over fears the party was making “unusual arrangements” with Breitbart.

Individuals paid by the right-wing US news organisation were working as senior unpaid Ukip volunteers, it is alleged.

The whistleblowers feared that work could be interpreted as an indirect political donation by a foreign donor, according to anonymous sources who spoke to The Guardian.

It is believed the complaints were brought to the attention of the Metropolitan police by the Electoral Commission, but the police decided to take no further action.

Now Ben Bradshaw, a former Labour Cabinet minister, has written to both organisations asking them to be transparent about the allegations.

In a letter to Ms Dick, he wrote: “I wondered if you could confirm the truth of this report claiming that the Electoral Commission referred impermissible foreign donations to UKIP to the Met in the run up to the EU Referendum, but that no further action was taken.

“If that was the case, I would also be interested in the reason for the decision.”

Mr Bradshaw has also asked the Commission if it knew if the Met had investigated and “decided not to take any action”.

The move follows the former minister demanding that the Government investigate the role that “dark money” may have played in the EU referendum.

Arron Banks, a former UKIP donor and ally of Nigel Farage says he contributed almost £9m in cash, loans and services to the Brexit campaign.

But the Open Democracy group alleged he is worth much less than the £100-250m he claims - raising concerns about where his pro-Brexit money may have come from.

The Breitbart volunteers would have worked for Ukip when its chief executive was Steve Bannon, before he became President Trump’s campaign adviser and, later, White House strategist.

Mr Farage praised Breitbart for its support of Brexit, saying its “supportive voice” was key to the Leave campaign winning the 2016 referendum.

The Guardian said its sources believed Breitbart had a “deliberate strategy” to wield influence over Ukip, to drive it to more right-wing positions, including on migrants.

The website is a platform popular among extreme white nationalists in the US, who have been bolstered by President Trump's rise to power.

There is no evidence that Ukip or Breitbart broke the law. A spokesman for Mr Farage has not responded to requests for comment.

Meanwhile, Mr Banks’ spokesman has described his businesses as profitable and sustainable, and that the former donor “broadly agrees” with a £250m estimate of his fortune.

It is illegal for British campaigns to accept foreign donations or donations from anyone who is not UK registered.

Volunteering is seen as an “interesting grey area” in the law that could be difficult to define, given most electoral campaigns are managed by volunteers.