Home Secretary Suella Braverman warned the public against flying over Christmas, after passport control officers voted to walk out, joining many sectors which have gone on strike this year as decades-high inflation erodes the value of earnings.

"If they go ahead with those strikes there will be undeniable, serious disruption caused to many thousands of people who have holiday plans," she told reporters.

The Border Force agency is training 2,000 soldiers to back up its personnel, officials said.

The Ministry of Defence is also in talks with the National Health Service, Downing Street said, after ambulance drivers voted to join nurses in going on strike this month.

"These sorts of rolling strikes will cause disruption for everyone," Prime Minister Rishi Sunak's spokesman told reporters, reiterating the government's refusal to countenance inflation-busting pay deals.

"And that does also include our military personnel who will be required, unfortunately, to have to step in and backfill some of these vital roles that we need to help keep the country moving."

Sunak on Wednesday hinted that ministers were preparing new legislation to outlaw strikes in critical sectors.

Currently, only Britain's police, military and prison guards are barred by law from striking.

"What we're looking at is, are there other areas that we should include in that?" Education Secretary Gillian Keegan told BBC radio Thursday.

"Health would be one to look at and other areas of critical infrastructure," she said, while adding that the government had yet to look at banning strikes by teachers, who have gone on strike in Scotland.

- 'Christmas chaos' -

Ahead of a new wave of train strikes beginning next week, rail union leaders claimed the government blocked a deal to end their long-running dispute over pay and other conditions.

"Rail workers and the industry have been put in an impossible position by the Tory government," Frank Ward, interim general secretary at transport union TSSA, said Thursday.

"Christmas chaos and disruption across our railways are now unfortunately guaranteed, and come gift wrapped from Rishi Sunak and his anti-worker Conservative government's agenda."

The government is pushing through a law to require a "minimum service" on the railways and has refused to rule out expanding that bill to encompass other public sectors.

The biggest UK strikes in 2022 have involved tens of thousands of railway and postal workers.

British inflation stands above 11 percent, the highest level in more than 40 years.

"I don't think those going on strike want strikes and disruption," Keir Starmer, the leader of Britain's main opposition Labour party, said Thursday.

"They are facing a very real cost of living crisis, they are struggling to pay their bills," he told business leaders.