SYDNEY (AFP) - British Foreign Secretary Liz Truss offered her "100 per cent" support for embattled Prime Minister Boris Johnson on Friday (Jan 21), backing him to remain in office "as long as possible".

Ms Truss - who has been tipped as a front runner to replace Mr Johnson - said she believed the 57-year-old Tory leader was doing an "excellent job".

A defiant Mr Johnson is fighting calls from within his own party to resign following revelations that Downing Street staff partied while the rest of the country was in strict Covid-19 lockdown.

"The Prime Minister has my 100 per cent support. He is doing an excellent job," Ms Truss said at a press conference with Australian counterparts in Sydney.

"I want the Prime Minister to continue as long as possible in his job. He is doing a fantastic job. There is no leadership election," she insisted. "The Prime Minister apologised and said mistakes were made."

Ms Truss dodged questions about new allegations that Mr Johnson's aides had blackmailed and threatened party members to ensure their backing.

"As you will appreciate, I have spent the last 48 hours here in Australia," she said. "I do not have any information about what is happening on the subject you raised."