* Johnson attended birthday gathering in lockdown
* Police will investigate alleged lockdown breaches
* Government's own internal inquiry to continue
LONDON, Jan 25 (Reuters) -- British police said on Tuesday they would investigate allegations of lockdown parties at Boris Johnson's Downing Street official residence, as media said an internal report that could determine his future might be published very soon.
Johnson is fighting for his political life after allegations that he and staff partied at the heart of the British government in breach of rules they had themselves imposed to fight the coronavirus pandemic.
Revelations of revelry including boozy parties in Downing Street, suitcases of supermarket alcohol, a broken children's swing, a wine fridge and jokes by staff about how to present such parties to reporters, have hammered Johnson's ratings.
ITV reported on Monday that Johnson and his now wife Carrie had attended a surprise party of up to 30 people for his birthday in the Cabinet Room at No. 10 Downing Street on June 19, 2020, when indoor gatherings were banned.
Johnson's office described the alleged party as a brief gathering by staff to wish him a happy birthday, adding that he was there for "less than 10 minutes."
A spokesman said Johnson did not believe he had broken the law over any of the gatherings.
"I welcome the Met's (London Metropolitan Police) decision to conduct its own investigation because I believe this will help to give the public the clarity it needs and help to draw a line under matters," Johnson told parliament.
Met Commissioner Cressida Dick, Britain's top police officer, said an investigation had been opened into a number of events "at Downing Street and Whitehall in the last two years."
Police had initially refused to look at the gatherings.
The Cabinet Office said its own investigation, by senior official Sue Gray, was continuing and there was ongoing contact with the police.
Johnson's spokesperson said Gray's team were discussing with the police whether she could still release her report in full before their investigation had been completed, denying reports officials were seeking to prevent its publication.
A number of media outlets reported that Gray had concluded her report and was set to hand it to Johnson's office. ITV political editor Robert Peston said on Twitter it was possible the report could be published in full later on Tuesday or on Wednesday.
Gray's report is seen as crucial to Johnson's fate, and he and his ministers have said people should not reach any conclusions before reading it.
"We're not going to speculate on the outcome. It's for her to decide and we'll await that review," Deputy Prime Minister Dominic Raab told Sky News.
Reports of the gatherings have seen Johnson's ratings plunge, with much of the public and some of his 359 Conservative Party lawmakers calling for him to resign.
So far, there are fewer than the 54 lawmakers required to trigger a confidence vote that could result in a leadership contest, but patience is wearing thin.
"With the police now investigating, this nightmare gets even worse," senior Conservative David Davis, who has already called on Johnson to quit, said on Twitter.
Johnson has survived scandals throughout his career but his premiership, straddling both Britain's departure from the European Union and the worst pandemic for a century, has been defined by turbulence.
His 2019 plan to suspend parliament and force Brexit through was overturned by the Supreme Court. Eventually, to the delight of millions who changed their political allegiance to vote for him, he negotiated a divorce deal with the EU.
When the pandemic struck, he drew criticism for delaying shutting the country down. He nearly died from COVID-19 himself before recovering to oversee a world-leading vaccine rollout.
But reports that he and his staff were flouting the rules they imposed on the British public are testing Johnson's legendary ability to bounce back.
"You break the rules, you've got to go," said Ian Dowrich, 59, a builder from Brentwood, Essex, who said he had voted for Johnson.
Police chief Dick said police had not typically investigated every alleged lockdown breach, but that, after receiving some findings from Gray's inquiry, there were grounds to do so now.