Boris Johnson knew of Vote Leave’s overspend during the 2016 EU referendum, but appears to have failed to tell the authorities, according to explosive new claims from a senior MP. The payment was subsequently ruled to be illegal.

Ian Lucas revealed that he has seen correspondence obtained during the parliamentary inquiry into disinformation and democracy which showed that Johnson’s most senior aide, Dominic Cummings, told the Electoral Commission that the prime minister, and his cabinet colleague Michael Gove, knew of the overspend by the pro-Brexit organisation.

The Electoral Commission last year judged that Vote Leave had broken electoral law by overspending during the EU referendum, after the campaign funnelled £675,000 through another pro-Brexit group, BeLeave, to avoid spending limits.

The revelation comes as it was confirmed that prosecutors have received a file of evidence from the Metropolitan Police that could lead to criminal charges against the Vote Leave campaign, which was spearheaded by Johnson and Cummings.

After nearly 16 months of investigating Vote Leave, the Metropolitan Police has handed a file of evidence to the Crown Prosecution Service and are seeking “early investigative advice” on how to develop the case against the pro-Brexit organisation. Vote Leave has always denied any wrongdoing.

Veteran Labour MP Lucas, who sat on the Digital, Culture, Media and Sport (DCMS) Committee inquiry into fake news, said the correspondence showing that the prime minister had apparently failed to report an offence raised serious questions over Johnson’s judgement.

Lucas told the Observer: “Johnson and Gove both knew about the illegal payments to BeLeave. I’ve seen it in writing. We finally forced the Electoral Commission to hand over its correspondence with Dominic Cummings; it’s there in black and white. It’s Cummings himself saying this.”

The Wrexham MP also revealed that the disclosure by Cummings was part of a strategy to protect the prime minister and Gove, with Johnson’s chief adviser insisting that both politicians only discovered the overspend after the EU referendum vote, though it’s unknown when exactly it is believed to be.

“The astonishing thing is, he actually said this in the context of defending them. He said they didn’t know at the time, they only knew after the referendum. That means he’s been sitting on this information the entire time,” said Lucas.

You’ve got two very senior politicians and they have never been required to give an account of what they knew Gavin Millar QC

He added: “It is totally unacceptable that the prime minister had knowledge that this overspend had occurred and he hasn’t come forward with this evidence,” said Lucas, who recently announced he would stand down at the next general election.

“What’s quite clear is Cummings, Johnson and Gove are absolutely in this up to their necks. They must now come clean about everything they knew about these offences. Boris Johnson is simply not fit to be prime minister. He clearly has no respect for the law,” Lucas said.

Gavin Millar QC of Matrix Chambers said: “You’ve got two very senior politicians in positions of great responsibility and they have never been required to give an account of what they knew. In the US they would have been hauled in front of a committee.”

The development arrives at a particularly delicate period for Johnson and Cummings with fresh pressure on Downing Street yesterday to release a potentially incendiary report into the extent of Russian interference in British politics.

Jeremy Corbyn accused Downing Street of delaying the report, which examined allegations that Kremlin-sponsored activity distorted the result of the 2016 EU referendum, asking Downing Street what the government “have got to hide”.