A cross-party group of parliamentarians is taking legal action against the Metropolitan Police over electoral offences in the 2016 EU referendum.

Lawyers for the group – which includes Liberal Democrat, Labour and Green MPs – have sent a legal letter to the Met demanding an explanation for delays in deciding whether individuals and groups associated with the Leave campaign should face criminal charges.

If no action is taken, the MPs warn they will seek judicial review of the force’s handling of a dossier of information handed over by the Electoral Commission last year.

In July 2018, the Commission imposed fines on the Vote Leave and BeLeave campaigns, as well as on BeLeave founder Darren Grimes, after finding that they breached electoral law by falsely reporting joint spending on the referendum. Another group, Veterans for Britain, was fined for incorrectly reporting a donation it received from Vote Leave.

Fronted by Boris Johnson and Michael Gove, Vote Leave was the designated lead campaigner for the Brexit camp in the referendum. The Commission found that Vote Leave passed a £680,000 gift to youth group BeLeave which would otherwise have taken the campaign’s spending over the £7 million limit. Vote Leave paid a £61,000 fine in March after dropping an appeal against the Commission's findings

The Met confirmed last September that it had received over 2,000 documents and a full explanation from the Electoral Commission on the breaches, but no announcement has yet been made on any possible criminal charges.

Liberal Democrat Brexit spokesman Tom Brake, one of the MPs behind the legal action, said: “The public are entitled to know without delay the extent of any criminal law-breaking that took place in the run-up, during and after the EU referendum campaign.

“Foot-dragging is not an option for the police when investigating claims relating to the integrity of our democracy.”

Lawyers Bindmans were due to deliver a letter before action to Met Commissioner Cressida Dick, challenging the force’s delay in investigation and seeking an explanation as to their failure to reach a charging decision.

A crowdjustice.com appeal for £12,000 to fund the legal action had raised £5,000 within a few hours of its launch.

Also backing the move are Labour’s MP Ben Bradshaw and ex-MP Fiona Mactaggart, as well as the Green Party’s Caroline Lucas MP and Baroness Jenny Jones.

They argue that the Electoral Commission has already stated that the electoral offences it detected “mirror” criminal offences under the same law.

The parliamentarians said in a statement: “The Metropolitan Police have been in possession of this information for over 11 months. To date, no decision has been reached as to whether any of these individuals should be charged.

“The test they have to consider is in two stages: Is there enough evidence? The answer must be a resounding yes, as the Electoral Commission has already found failings and irregularities. The second stage test is: Is it in the public interest to prosecute? We believe yes.

“This is necessary so that there is confidence in the rule of law and democratic processes of our country and so that the law is not broken with impunity.