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.
They will also go on strike at Birmingham, Cardiff, Glasgow and Manchester airports, as well as the Port of Newhaven.
Read more: Which industries are striking and when?
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.
Earlier this week, the government said 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.
The Cabinet Office 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.
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.