It comes as calls mount for a Downing Street aide to resign for allegedly “outing” the whistleblower who made the claim about improper co-ordination between Vote Leave and the BeLeave group.

Former BeLeave campaigner Shahmir Sanni alleges that Vote Leave used his organisation to get round the £7 million legal spending limit by handing over £625,000.

A two-hour emergency debate has been granted to allow the MPs to consider the EU referendum and alleged breaches of electoral law. Speaker John Bercow allowed the debate after MPs supported an application from Liberal Democrat Tom Brake.

Foreign Secretary Boris Johnson has dismissed the allegation as “ludicrous”, and Vote Leave have denied wrongdoing, insisting BeLeave was fully independent.

Mr Sanni claims Stephen Parkinson, a senior figure in the Vote Leave campaign who is now Theresa May’s political secretary, issued instructions to BeLeave and that the two organisations used the same cloud drive to store campaign materials.

The £625,000 was reportedly given to Canadian company AggregateIQ, which is alleged to have links to Cambridge Analytica, the data firm under investigation over the harvesting of 50 million Facebook profiles to micro-target messages for Donald Trump’s presidential campaign.

Mr Sanni also alleges that he was “outed” by No 10 in a press statement at the weekend over an 18-month relationship he had with Mr Parkinson.

Facing questions in the Commons yesterday, Mrs May stood by her aide after Labour former minister Ben Bradshaw said Downing Street’s conduct was “a disgrace”.

“How is it remotely acceptable that when a young whistleblower exposes compelling evidence of law-breaking by the Leave campaign, implicating staff at Number 10, one of those named instead of addressing the allegations issues an officially sanctioned statement outing the whistle blower as gay and thereby putting his family in Pakistan in danger,” Mr Bradshaw asked. “It’s a disgrace, Prime Minister, you need to do something about it.”

Mrs May said any statements issued by Mr Parkinson were “personal statements”.

Labour former minister Angela Eagle also homed in on Mr Parkinson, calling on the Prime Minister to sack him. Mrs May responded: “No, I’m sorry, that is not what I should be doing, my political secretary does a very good job.”

Mr Parkinson said he believed it was inevitable that details of their relationship would become known once Mr Sanni decided to go public with his allegations.

Lawyers for Mr Sanni and Chris Wylie, the Cambridge Analytica whistleblower who disclosed the Facebook data breach and a friend of Mr Sanni, say their evidence provides “reasonable grounds” for the Electoral Commission to investigate the claims.

A 50-page opinion obtained by Bindman’s solicitors and prepared by barristers from Matrix Chambers calls for an “urgent investigation” to establish whether a prosecution could be brought. It argues the Electoral Commission should investigate whether offences were committed “with the knowledge, assistance and agreement” of senior figures in Vote Leave, including Mr Parkinson and Cleo Watson, who also works in Number 10, as well as Vote Leave campaign director Dominic Cummings.

“Given the very close working relationships at all material times between Vote Leave and BeLeave, the way up which Mr Parkinson and Ms Watson supervised the work of the young BeLeave volunteers and that Vote Leave and BeLeave staff worked closely together, it can be properly inferred that Mr Parkinson and Mr Watson must have known about BeLeave’s campaign activity, of which the AIQ targeted messaging was a significant part,” said the opinion.

Theresa May facing claims Brexit campaign broke spending rules