Troops have been training at Heathrow and Gatwick airports for passport checking roles ahead of Border Force staff going on strike over Christmas.
The Ministry of Defence confirmed to Sky News armed forces personnel arrived at London’s two main airports earlier this week.
The PCS union announced on Wednesday that Border Force officers will go on strike at the airports from 23-26 and 28-31 December after they rejected a 2% pay rise offer from the government.
They will also go on strike at Birmingham, Cardiff, Glasgow and Manchester airports, as well as the Port of Newhaven.
About 75% of passport control staff are PCS members, meaning the majority of staff checking passports will be going on strike.
Read more: Which industries are striking and when?
The Cabinet Officer this week said up to 600 military personnel and 700 civil servants were being trained to support a range of services – including Border Force at airports and ports – in the event of strike action.
The Home Office has warned the strikes are likely to lead to longer queues at passport control at one of the busiest times of the year for airports.
Airports have advised travellers to check the status of their flights before travelling.
Prime Minister Rishi Sunak visited an RAF base on Friday where he thanked military personnel for stepping in and missing their Christmas.
“We all owe them an enormous debt of gratitude,” he said.
He added that his priority is to “protect lives and to minimise the disruption on people’s lives” but insisted the government will always “try and act fairly and reasonably” with public sector pay.
“What I’m not going to do is ask ordinary families up and down the country to pay an extra £1,000 a year to meet the pay demands of the union bosses. That wouldn’t be right and it wouldn’t be fair,” he said.
The use of troops to cover striking workers has proved contentious, with senior military figures saying they should not be made to give up Christmas.
Armed forces personnel are banned by law from striking themselves and most soldiers are already paid less than those going on strike, while their pay scales have not kept up with inflation.
A senior defence source told the Telegraph the government is now reaching for the Armed Forces “every time there is any difficulty, whether it’s floods, strikes…as opposed to it being the last resort”.
Ex-Army captain and Tory MP Tobias Ellwood, chair of the Defence Committee, said it would place a “huge burden” on the military.
Earlier this week, the government confirmed military personnel, civil servants and volunteers are being trained to support a range of services as several industries will be hit by strike action this December.
On Sunday, Conservative Party Chairman Nadhim Zahawi told Sky News the government had yet to decide on whether to deploy armed forces personnel but said it was considering having them work on UK borders.
Military personnel were previously deployed to drive petrol tankers and deliver COVID jabs during the pandemic.
Industries also going on strike during the Christmas period are: rail workers, bus staff, roads staff, baggage handlers, Royal Mail employees, nurses, driving examiners, civil servants, ambulance staff and teachers.