A flight to bring about 200 British nationals back to the UK from coronavirus-hit Wuhan is unable to take off as planned on Thursday.
It is understood relevant permissions from Chinese officials have not yet come through.
The Foreign Office said it was "working urgently" to organise a flight to the UK "as soon as possible".
The virus has caused at least 170 deaths, spreading to every Chinese region and at least 16 other countries.
The flight from Wuhan, the city where the virus first emerged, had been expected to arrive at RAF Brize Norton in Oxfordshire on Thursday morning. Passengers were to be taken to an NHS facility.
When the passengers do return they will be put in "supported isolation" for 14 days with "all necessary medical attention", a Downing Street spokesman said.
A small team of military medics is flying out to Wuhan to accompany passengers on the repatriation flight, the Ministry of Defence said.
Britons waiting for the flight said they had been told it had been put back "provisionally" to Friday.
Jeff Siddle, from Northumberland, told the BBC his family faced a "terrible dilemma" after he and his nine-year-old daughter were told they could fly back - but not his Chinese wife, who has a permanent residency visa for the UK.
"Weve been told were only allowed one hand luggage item, so weve basically had to virtually ditch all of our big suitcases and all of our holiday clothes and take just what we think we absolutely need.
"Were living out of what we can, with all our bags packed," he said.
As it stands, Mr Siddles wife is not travelling back to the UK with him. "I think theres still negotiations to get that changed, but the official line is still that she cant fly," he said.
BBC China correspondent Robin Brant said the issue seemed to be that the Chinese government was not giving permission for its citizens to leave the country.
The Foreign Office was "optimistic" of sorting the problem, he said, adding that the hope was the flight could take off Friday morning local time and that Britons and their partners would be on it.
Ben Pinkerton, an English teacher from Northern Ireland who was also due to board the flight, said he was "sitting tight".
"We feel fine, weve isolated ourselves. We wore the masks when we went out," he said.
Chinese health authorities said there were 7,711 confirmed coronavirus cases in the country as of Wednesday night.
The Foreign Office said a number of other countries flights had also been unable to take off as planned.
"We remain in close contact with the Chinese authorities and conversations are ongoing at all levels," a spokeswoman said.
Stress of being locked up
Hundreds of foreign nationals are being evacuated from Wuhan, with Japan, the US and the EU among those repatriating their citizens.
Some 200 Japanese nationals have been flown from Wuhan and have landed at Tokyos Haneda airport.
Roughly 200 Americans - including workers from the local US consulate - have also left Wuhan on an evacuation flight.
However, a number of UK citizens in China said local authorities had prevented members of their families from leaving because they held Chinese passports.
Another UK citizen due to be flown out of Wuhan said she was told to leave behind her three-year-old son because he has a Chinese passport.
Natalie Francis, originally from York, said: "I literally had no words when I got the call."
"All three of us are in Wuhan at the moment," she added. "Physically we are fine, but the stress of being locked up for so long... the emotional health is starting to suffer a little bit... especially after this news."
The Foreign Office said its priority was to keep British nationals and their families together.
The World Health Organization (WHO) will decide later whether the virus constitutes a global health emergency.
It is a basic question, but the answer is elusive.
It is far too simplistic to take the 170 deaths and the 7,711 cases and come up with a death rate of 2%.
We are in the middle of the outbreak and thousands of those patients are still being treated. We dont know if they will live or die, so they cant be used in these calculations.
We also dont know how many mild and undetected cases are out there.
Also, the deadliness of the new virus is only one component of its threat.
Flu kills hundreds of thousands of people each year, not because it is super-deadly, but because it is able to infect so many people.
"In the last few days the progress of the virus, especially in some countries, especially human-to-human transmission, worries us," WHO Director-General Tedros Adhanom Ghebreyesus said on Wednesday, pointing to Germany, Vietnam and Japan.
"Although the numbers outside China are still relatively small, they hold the potential for a much larger outbreak."
Professor Sian Griffiths said the "shared response across the world" to the new coronavirus was more effective than during the 2003 SARS outbreak, which she examined in an official inquiry.
She told BBC Breakfast that world governments were "taking everything very seriously in an attempt to contain the virus", deploying greater resources and offering clear public health messages.