Two-thirds of UK adults have now been fully vaccinated against the coronavirus, the government said Wednesday, arguing that the breakthrough justified its contentious move to drop most pandemic curbs next week.

A total of 35,155,767 second doses have now been administered since the Western world's first mass vaccination rollout began in December, official figures showed.

"Barely 8 months since the first vaccine, 2/3 of UK adults have had both doses," Prime Minister Boris Johnson tweeted.

"Thank you to everyone coming forward and to those helping others get jabbed. You are the reason we are able to cautiously ease restrictions next week," he said.

From Monday, the government will lift most restrictions on public gatherings in England and allow businesses such as nightclubs to reopen, to the concern of scientists who say it is too soon with the Delta variant still spreading rapidly.

There will be a patchwork of different rules on face coverings across the UK and within England, if other cities and transport operators follow the lead of London mayor Sadiq Khan.

Although the government is lifting the legal mask requirement in England, Khan said it would remain obligatory for all passengers on London's transport system, including buses and Underground trains.

"I've repeatedly made clear that the simplest and safest option would have been for the government to retain the national requirement for face coverings on public transport," the mayor said in a statement.

"I'm not prepared to stand by and put Londoners and our city's recovery at risk."

Transport Secretary Grant Shapps said Khan's move was "very much in line" with the UK government's wishes, despite Johnson's insistence that personal responsibility should now take precedence over legal diktat.

The UK's other nations, Scotland, Wales and Northern Ireland, set their own health policy and are moving more slowly.

Welsh First Minister Mark Drakeford on Wednesday detailed a plan to progressively lift the remaining restrictions by August 7, but it will remain law to wear a face mask on public transport and in most indoor settings in Wales.

Scotland is taking a similarly phased approach and plans to scrap most of its legal COVID restrictions on August 9, but will also retain the masks mandate.

"We shouldn’t lift important restrictions to make our lives easier and then expect the public to take responsibility for doing the right thing,” Scottish First Minister Nicola Sturgeon said on Tuesday.

Downing Street denies that Johnson has caved to Conservative libertarians who have long chafed at the requirement for masks and other restrictions.