The Foreign Office is investigating reports that two Britons were on a plane that disappeared flying over the jungle in Cameroon.
The plane went missing on Saturday during a flight to the Republic of Congo.
An FCO spokesman said: "We are investigating reports that two Britons are missing.
"Our High Commission in Cameroon is in close contact with their Australian counterparts."
The aircraft, carrying nine passengers, was chartered by Australian mining firm Sundance Resources.
Six of them have been named and identified as Australians who work for the firm but the nationality of the other three is unknown.
A spokesman for the company said a search is under way.