The Metropolitan Police has refused to release details of its decision not to investigate allegations of political corruption.

A ‘cash for peerages’ investigation by openDemocracy and The Sunday Times in November revealed a pattern of high-value Tory donors securing seats in the House of Lords. Opposition MPs urged Met commissioner Cressida Dick to investigate, but the force refused, saying there was “insufficient information”.

Now the Met is also refusing to release any information about how it came to that conclusion – including copies of internal emails and whom it consulted.

Responding to Freedom of Information (FOI) requests from openDemocracy, the force claimed that transparency about its failure to look into the scandal could cause it “operational harm”.

“Such a disclosure would not be in the public interest,” it said. “It would be harmful to our policing functions.”

It comes amid renewed pressure over the Met’s investigation into Downing Street parties.

After weeks of refusing to investigate ‘partygate’, the force announced on Wednesday – as Sue Gray’s report on the alleged parties was due to be published – that it would launch an inquiry after all.

Then, this morning, the force said it had asked the Cabinet Office to remove any details about potentially illegal events from Gray’s report – saying anything more than “minimal reference” could prejudice its inquiries.

Experts have questioned whether there is any legal basis for this. One MP, Liberal Democrat Alistair Carmichael, warned it would be “profoundly damaging” if there was any hint of an “establishment stitch-up”.

Labour leader Keir Starmer has accused Boris Johnson of using the police as a “shield” to protect his position.

As pressure on the Met mounts, the force is also facing legal action for its failure to launch an inquiry into the peerages scandal.

The investigation by openDemocracy and The Sunday Times found Conservative Party treasurers who donate at least £3m are overwhelmingly likely to be given a seat in the House of Lords. In the past seven years, all except for the most recently retired party treasurer have been offered a peerage after donating this amount.

One former party chairman admitted: “Once you pay your £3m, you get your peerage.”

Analysis suggests the odds of so many Conservative donors in the UK population all being given a seat in the House of Lords is equivalent to entering the National Lottery 12 times in a row and winning the jackpot every time.

Several formal complaints were made following the revelations. In a letter to Cressida Dick, the SNP’s Pete Wishart urged: “These widespread allegations and suspicion of criminal activity need to be urgently addressed.

“The evidence, I believe, must focus on the evidence uncovered by the openDemocracy website and The Sunday Times newspaper.” He added that the scandal was “deeply undermining public trust”.

Today, Wishart said the police's failure to investigate had been "disappointing". But he told openDemocracy: "Now that they have opened an investigation into alleged parties within No. 10, there is no reason why they should not reconsider investigating it."

The police previously investigated a ‘cash for honours’ scandal in 2006 and 2007, when Tony Blair was in office. He became the first serving prime minister to be questioned by police conducting a criminal investigation – although he was never interviewed under caution or arrested.

Last year, the Met Police were accused of “serious failure” after openDemocracy revealed the force was labelling FOI requests as ‘high profile’” if they concerned sensitive issues or were sent by journalists. Transparency rules say FOIs should be ‘applicant-blind’.

At the time, the former shadow chancellor John McDonnell described the practice as “shocking”. He added: “The effective operation of Freedom of Information is critical to ensuring we have accountable public services.”