Thomas Cook is evacuating nearly 1,000 UK customers home from The Gambia following a change in Foreign Office (FCO) travel advice.
A 90-day state of emergency has been declared in the country, which the FCO fear could lead to a "high" risk of military intervention and civil disturbance.
Gambia's president of 22 years, Yahya Jammeh, has refused to hand over power after losing an election last year.
Senegal troops are at the Gambian border and are poised to enter the country if no political solution is reached by midnight, a senior Senegalese military official told Reuters.
Witnesses have confirmed they have seen Senegalese troops moving towards the Gambian border.
The holiday company said it will operate an additional schedule of flights from the capital Banjul and bring 985 package holiday customers back to the UK over the next 48 hours.
British tourists were informed of their immediate evacuation at various holiday resorts and hotels in The Gambia on Tuesday.
The West African country has become an increasingly popular destination for winter sun holidays.
Thomas Cook said: "We will operate a programme of additional flights into Banjul airport over the next 48 hours to bring the 985 UK customers we currently have on holiday in The Gambia home, including four additional flights on Wednesday 18 January.
"In addition, we have approximately 2,500 flight-only customers in Gambia, whom we are contacting to offer the earliest possible flight availability for return to the UK."
Hundreds of passengers have been pictured queuing up to leave in the airport.
Caroline Cruddas, 53, and her 19-year-old son Brandon, from Grimsby, said they had been due to fly back to Manchester on Wednesday after a three-week holiday in the troubled west African state.
But the pair, who are among around 2,500 Thomas Cook flight-only customers in The Gambia, said they had been told their flight had been cancelled because they were not on a package deal - even though other flights were leaving on Wednesday.
Mr Cruddas said they had been to The Gambia 12 times but they are now "uneasy" and "worried about our safety".
He added: "Knowing we're not going home is very scary and we pray we're on that plane tomorrow."
Thomas Cook said the plane was not cancelled but there appears to have been a "miscommunication".
The firm said: “We’re very sorry and we are working hard to get them home as soon as possible.”
The Gambia's parliament has extended President Yahya Jammeh's term, allowing him to stay in power for three months.
His term was due to end on Thursday following his defeat in elections and opposition leaders had threatened to use force to oust Mr Jammeh if he refuses to hand over power.
On Tuesday he banned "any acts of disobedience" and ordered security forces to maintain order.
Hundreds of Gambians have also been fleeing over the border to Senegal in the last few days, fearing civil unrest.
Travel association ABTA has estimated that there are approximately 2,000 people using UK travel companies in The Gambia and more who have travelled independently for holidays or to visit friends and family.
- What is the latest travel advice?
The Foreign and Commonwealth Office (FCO) advise against "all but essential travel to The Gambia due to the deteriorating political situation and potential military intervention following the Presidential elections on December 1."
People currently in The Gambia should leave by commercial means if there are no essential reasons to remain, the FCO said.
It added that the Economic Community of West African States (ECOWAS) may instigate military action if President Jammeh does not step down on the scheduled handover date of January 18.
"The potential for military intervention and civil disturbance is high and could result in Banjul International Airport being closed at short notice," the FCO said in a statement.
It advised travellers currently in The Gambia to:
- Follow events closely and take extra care of safety.
- Keep in touch with travel operators and airlines to monitor travel advice.
- Avoid large crowds.
- Avoid discussing politically sensitive topics in public.
- Expect vehicles to be searched by security at checkpoints in and around the capital.
- Make contact with family and friends to inform them of travel arrangements.
- Monitor the FCO website for updates on The Gambia.
ABTA said package holidaymakers should contact their tour operator to confirm arrangements for returning home and independent travellers should contact their airlines to discuss their options.
It said that insurance policies for people travelling to The Gambia will be invalidated but cover for travellers already in the country will remain in place.