Unprecedented travel chaos across country could continue for at least another six months unless the government makes workers an improved pay offer, unions have warned.
Mark Serwotka, the general secretary of the PCS union, which represents striking Border Force staff, also predicted there would be a "huge escalation" in other industrial action next month unless ministers enter into negotiations.
The rail dispute will also continue in the new year unless ministers act, the Rail, Maritime and Transport union (RMT) said.
The warnings came as the prime minister was challenged to explain “where have you been” after a week of walkouts including by ambulance staff and nurses. Rishi Sunak was also forced to caution people to check before they set out for their destination.
With the great Christmas getaway underway, Mr Sunak said: "I would urge everybody who is travelling at the moment to just please check before you make your journey - so you know what it happening."
Mr Serwotka predicted that today’s action at the border, which is expected to affect travellers both sides of Christmas, “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 the BBC Radio 4 Today programme.
If not, he said, “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.”
Up to 100,000 civil servants could strike after Christmas and it will be "inevitable" that previously unaffected areas such as the Port of Dover will be hit by industrial action if the Government refuses to talk, he added.
"It 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."
He also said he had shown a government minister examples of his members' poverty, but was told "not a single penny would be put on the table" because the government cannot afford it. But, he added, “nobody believes that”.
Visiting a homeless shelter in London, Mr Sunak defended his government’s decision to hold firm on pay in the face of high inflation.
Not negotiating on wages is "in the long term… the right thing for the whole country", he said. "What I'm trying to do is make the right long-term decisions for the country, for everybody's benefit.
"We all know the major economic challenge we all face now is inflation, it's inflation eating into everyone's pay packets...
"I want to make sure we reduce inflation, part of that is being responsible when it comes to setting public sector pay. That's why we have an independent process.
"I know things are difficult but it's right there's an independent body that makes recommendations to the Government and the Government accepted those. It increased its offer, matched all those recommendations, I think that's the reasonable thing to do. And in the long term it's the right thing for the whole country that we beat inflation."
The government is now facing a series of rolling strikes on multiple fronts. 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 on Thursday.
Postal workers represented by the Communication Workers Union walked out for their fifth day of action this month, in a move Royal Mail said was a "cynical attempt to hold Christmas to ransom".
The company said it would be doing all it could to deliver Christmas mail, saying the industrial action has cost it £100 million.
RMT railway workers will stage another strike from 6pm on Christmas Eve, which could prevent travellers from making it home for Christmas.
