London police is investigating alleged lockdown breaches at Prime Minister Boris Johnson's Downing Street residence after receiving evidence from an internal government probe into a series of gatherings, according to ndtv.

Johnson is fighting for his political survival after new allegations that he broke the COVID-19 lockdown rules he imposed by attending a surprise birthday party in Downing Street when social gatherings indoors were banned.

That added to a long list of alleged lockdown breaches in Downing Street, including one 'bring your own booze' party, which Johnson has said he attended thinking it was a work event.

The Met is now investigating a number of events that took place at Downing Street and Whitehall in the last two years in relation to potential breaches of Covid-19 regulations, London's Metropolitan Police Commissioner Cressida Dick said on Tuesday.

"I can confirm that the Met (Metropolitan Police) is now investigating a number of events that took place at Downing Street and Whitehall in the last two years in relation to potential breaches of COVID-19 regulations," she said.

The police investigation could force a delay to the internal inquiry commissioned by Johnson and carried out by senior official Sue Gray, who is expected to report her findings later this week in what was seen as a key moment for Johnson's future as prime minister.

About the allegation, Downing Street said Johnson, who turned 56 on June 19, 2020, was present at the event "for less than 10 minutes" as his staff "gathered briefly" to wish him a happy birthday.

The strict lockdown rules imposed at the time to contain the spread of coronavirus banned most indoor gatherings involving more than two people. But ITV News' reported on Monday night that up to 30 people attended the event, sang Happy Birthday and were served cake.

The event reportedly took place in the Cabinet Room of Downing Street just after 2 pm local time that day and had been arranged as a surprise for Johnson by his then-fiancee and now wife, Carrie Symonds, after he returned from an official trip to a school in Hertfordshire.

"A group of staff working in Number 10 that day gathered briefly in the Cabinet Room after a meeting to wish the Prime Minister a happy birthday. He was there for less than 10 minutes," a Number 10 Downing Street spokesperson said.

The ITV report also claimed that the same evening, family friends were hosted upstairs in the Prime Minister's official residence in apparent breach of the rules.

However, Number 10 denied that claim: "This is totally untrue. In line with the rules at the time the Prime Minister hosted a small number of family members outside that evening."

A number of his ministers spoke out in his defence even as the rebellion among backbench MPs continues to mount.

"It obviously was the Prime Minister's birthday, he'd been given a cake earlier in the day, that's the picture in the newspapers," UK Transport Secretary Grant Shapps told Sky News'.

And he was clearly given a cake by staff when he got back to the office. These are staff he would have been working with and was working with all day long, and will have been many a time in the same room with them working on the response to coronavirus. They come in, give him a cake, I understand I think it lasted for 10 minutes and that was it," he said.

The minister reiterated that it was ultimately for Sue Gray, the senior civil servant leading the Cabinet Office inquiry into all partygate allegations within government, to decide whether this was appropriate.

"I think we can be pretty clear that the Prime Minister didn't present the cake to himself, he added.

UK Culture Secretary Nadine Dorries saying in a tweet: "So, when people in an office buy a cake in the middle of the afternoon for someone else they are working in the office with and stop for 10 minutes to sing happy birthday and then go back to their desks, this is now called a party?"

But Opposition Labour Party leader, Sir Keir Starmer, repeated his call for Boris Johnson to resign in the wake of the latest revelations.

The Prime Minister is a national distraction and he's got to go," he said.

It comes as Johnson meets his Cabinet for a regular weekly meeting on Tuesday amid the growing threat of war with President Putin in eastern Europe, against the backdrop of the partygate scandal.

Meanwhile, Sue Gray's much-anticipated report into the scandal is expected by the end of this week and, according to BBC reports, she was already aware of the June 19 birthday event.

A Conservative backbench rebellion against Boris Johnson's leadership is likely to continue intensifying until then. A total of 54 Tory MPs must write letters of no confidence in Johnson as a leader to Sir Graham Brady, chairman of the party's powerful backbench 1922 Committee, to trigger a leadership contest. Most are said to be waiting for the inquiry report to make up their minds.