Military personnel checking passports at UK airports appear to be ensuring passengers are not being delayed on the first day of Border Force strikes.
Heathrow and Gatwick both said their immigration halls were operating as normal this morning.
Around 1,000 members of the Public and Commercial Services (PCS) union employed by the Home Office to operate passport booths walked out at Heathrow, Birmingham, Cardiff, Gatwick, Glasgow and Manchester airports, as well as the port of Newhaven in East Sussex.
The Border Force strikes will take place every day for the rest of the year, except 27 December.
About 250,000 passengers are arriving on flights at affected airports today, including around 10,000 who landed at Heathrow before 7am.
Travellers were warned by Border Force bosses to expect delays amid fears long queues at passport control could lead to people being held on planes, disrupting subsequent departures.
But the use of military personnel trained to check passports appears to be preventing delays.
A Heathrow spokeswoman said: "The morning arrivals peak has started well.
"Immigration halls are free-flowing at Heathrow with Border Force and the military contingency providing a good service."
A spokesman for Gatwick said: "Everything is going OK at the moment.
"There's plenty of staff. The e-gates are all operating. It's going well.
"There's no delays as far as we're aware, and no queues at the moment.
"I'm standing in arrivals and passengers are flowing through as normal."
Meanwhile, postal deliveries and driving lessons are set to be disrupted by strikes in the days before Christmas.
Royal Mail, National Highways and Driver and Vehicle Standards Agency (DVSA) employees are taking industrial action today.
The Royal Mail criticised the strike as "a cynical attempt to hold Christmas to ransom".
The company said it will be doing all it can to deliver Christmas mail, saying the industrial action has cost it £100 million.
The strike will continue into tomorrow when staff represented by the Rail, Maritime and Transport (RMT) union, Abellio London bus workers and Environment Agency workers will launch separate waves of action.
National Highways workers responsible for motorways and major A-roads in London and the South East, also represented by the PCS, will continue their four-day walkout, which started yesterday.
RMT railway workers will stage another strike from 6pm on Christmas Eve, which could prevent people from making it home for Christmas.
PCS general secretary Mark Serwotka said travellers could face months of disruption unless the government comes forward with an improved pay offer for Border Force staff.
"We think that the action at the borders is going to be very effective. We hope that the government will therefore do the right thing and get around the negotiating table and put some money upfront," he told BBC Radio 4's Today programme.
"If not, we are raising money. We have a strike fund that means we can sustain this action. Our strike mandate lasts right up until May. We will be supporting this action up to May and we would re-ballot again if we have to.
"I think in January what you will see is a huge escalation of this action in the civil service and across the rest of our economy unless the Government get around the negotiating table."
It follows two days of strikes by NHS staff, with thousands of nurses walking out on Tuesday and ambulance workers joining picket lines on Wednesday.
Nurses will also strike again on 18 and 19 January, with further dates to be confirmed in the new year, their union said this morning.
The new dates from the Royal College of Nursing comes days after British health sector bosses warned the government of the risk to patient safety from a wave of industrial action.
Union calls off planned ambulance worker strike
However, a planned post-Christmas strike by ambulance workers in the GMB union has been suspended.
Members of the union were due to walk out on 28 December in an increasingly bitter dispute over pay and staffing.
The GMB said there was "incredible" support during industrial action by the union and members of Unite and Unison on Wednesday.
Rachel Harrison, GMB national secretary said: "We are overwhelmed by Wednesday's amazing public support for our paramedics and ambulance staff.
"People across the country have been wonderful in backing us and we care so much about them too.
"That's why we are suspending the proposed GMB industrial action on December 28.
"We know the public will appreciate being able to enjoy Christmas without any additional anxiety. They support us and we support them."
She added: "The workforce crisis in our NHS is so severe and our commitment to getting ambulance staff the proper pay they deserve is stronger than ever, so we are scheduling a further date for action on January 11 2023.
"The incredible British public are why we are suspending our action over the Christmas period.
"But it also means the government can now do what ambulance workers and the public want - get round the table and talk pay now.
"We are here 24/7. Any time, any place.
"Over to you, (Health Secretary) Steve Barclay. Everyone is waiting."
