A senior Conservative MP has demanded that Scotland Yard urgently explain why it has not opened a criminal investigation into three pro-Brexit campaigns that the Electoral Commission found had broken the law.

Damian Collins, chair of the Commons committee investigating the illegal use of data during the EU referendum, told the Observer he was concerned that the Metropolitan police had as yet failed to launch a formal investigation into potential crimes committed by pro-Leave groups before the 2016 referendum.

His intervention comes five months after the official election regulator ruled that criminal offences had been committed and the force was first handed a dossier containing evidence of the potential crimes.

On Thursday, the website Open Democracy reported that the Met had stalled the launch of a criminal investigation into the pro-Brexit campaigns citing “political sensitivities”.

An email seen by the Observer from Open Democracy shows a Met press officer using the phrase to explain why a formal investigation had not been opened, with the explanation that this would “relate to ANY allegation or referral relating to an election”.

Hours after the report appeared, the Met issued a statement saying it had received more than 900 documents from the Electoral Commission, which are “being assessed in order to make an informed decision as to whether a criminal investigation is required”.

Collins, the chair of the Commons digital, culture, media and sport committee, said the report was troubling. “If the law was broken they [the police] have a duty to investigate. It makes no sense at all. I have no idea what’s going on. We need a proper official response,” he said.

In May and July this year, the Electoral Commission reported that false declarations, multiple breaches of electoral law and covert campaign overspending had taken place by pro-Leave groups during the referendum. Collins added: “What is our law worth if the police won’t act?”

Damian Collins, the chair of the digital, culture, media and sport commitee, is concerned there is no police investigation. Photograph: Andy Hall/The Observer

He added that Scotland Yard’s lack of apparent action appeared in stark contrast to its approach when it raided the parliamentary offices of former first secretary of state Damian Green in 2008 during a police investigation into a series of leaks from inside the Home Office.

“They weren’t so worried about ‘political sensitivities’ when they kicked down Damian Green’s door,” said the MP for Folkestone and Hythe.

The Electoral Commission has made two referrals to the Met regarding potential criminal offences – the first received on 11 May concerns Leave.EU. The organisation was fined £70,000 for overspending by at least £77,380 during the campaign with its chief executive, Liz Bilney, referred to the police. Leave.EU co-founder Arron Banks accused the commission of pursuing a “ridiculous witch hunt”.

The second referral was received on 17 July and concerned Vote Leave, the official campaign fronted by Boris Johnson and Michael Gove, which was fined £61,000 in July after it was found to have funnelled more than £500,000 in donations to another group, BeLeave.

Vote Leave has stated that it had “provided evidence proving there was no wrongdoing” and was “confident these findings will be overturned”.

Collins also revealed he was baffled about the lack of apparent progress by the National Crime Agency, which has received evidence from the Electoral Commission concerning the main pro-Brexit campaigns.

“We don’t know. We’re no clearer on that. We now have different government agencies who know about this. We know there is evidence of attempts to interfere in British politics. Why is there not a broader investigation?” said Collins.

The MP said it was “absurd” that the UK had not reacted to claims of Russian interference in the referendum by replicating the investigation of US special counsel Robert Mueller, who is heading an inquiry into alleged Russian meddling in the 2016 US presidential election. “In America, where there are deep political divisions in Congress, they still agree on the importance of this and the need for a special prosecutor. We need the same,” said Collins.

He also warned Mark Zuckerberg that the Facebook chief’s refusal to assist the parliamentary inquiry into the misuse of data should result in the UK’s security services being allowed to investigate Facebook.

“We haven’t even scraped the surface of what happened on Facebook. We are totally reliant on Facebook’s internal investigations. It’s not enough. The government needs to order the security services to go in and work with Facebook,” said Collins.

• The headline of this article was amended on 14 October 2018 to relate more closely to its contents, an MP’s call for an explanation of the lack of a criminal investigation, rather than the data misuse itself.