Britain postponed the planned evacuation on Thursday of around 200 of its citizens from Wuhan, the Chinese city at the epicentre of a deadly virus outbreak.
London has chartered a plane to fly out nationals stranded in the central city, and had said it would leave on Thursday.
But AFP understands British officials have not yet received permission from China for the flight to depart.
"We are doing everything we can to get British people in Wuhan safely back to the UK," a foreign office spokeswoman said.
"We continue working urgently to organise a flight to the UK as soon as possible.
"We remain in close contact with the Chinese authorities and conversations are ongoing at all levels."
The Wuhan metropolis is at the heart of the coronavirus epidemic that has so far killed 170 people and spread to at least 15 countries around the world.
China reported its biggest single-day jump in new coronavirus deaths on Thursday, deepening global fears about the outbreak.
Thousands of foreigners have been trapped in Wuhan since it was sealed off last week.
Japan and the United States became the first countries to organise airlifts out of Wuhan for their citizens on Wednesday.
Britain is joined by Australia, New Zealand and Singapore, among others, in planning similar operations.
London on Wednesday said those returning will be "safely isolated" for two weeks with all the necessary medical attention.
