British Prime Minister Boris Johnson braced Wednesday for a potentially damning report into lockdown-breaching parties, after the launch of a police inquiry dramatically upped the stakes.

The report from an investigation by senior civil servant Sue Gray is complete, but has not yet been submitted to Downing Street, the government said.

The explosive confirmation that London's Metropolitan Police force has now started its own investigation could complicate the release of Gray's report, but opposition parties insisted on its publication in full.

Otherwise, according to senior Labour MP Chris Bryant on BBC radio, the public will "simply say this is a cover up".

It was unclear whether Downing Street would release the report later yesterday or some other time, with Johnson facing another grilling in parliament at the weekly joust of Prime Minister's Questions from noon. He again yesterday refused to resign as opposition MPs heaped pressure on him.

At least seven backbench Conservative MPs have called publicly for Johnson's resignation. A total of 54 letters are required to trigger a party vote of no confidence.