The BBC says it is concerned for the safety of a correspondent who has gone missing in the Gaza Strip.
Johnston has been the BBC's reporter in Gaza for three years
The corporation said it had been unable to contact Alan Johnston, but did not comment on Palestinian reports that he had been kidnapped.
Johnston's car was found abandoned in Gaza City shortly after he left his office to drive home.
Several journalists and aid workers have recently been kidnapped in Gaza. All have been released unharmed.
Johnston, 44, has been the BBC's correspondent in the Gaza Strip for the past three years - and the only foreign journalist from a major media organisation based in Gaza.
"We are currently unable to contact him and are concerned for his safety," the BBC said in a statement.
"We are trying to gather as much information as possible."
The BBC's Matthew Price in Jerusalem says all the main Palestinian militant groups have called for Johnston's release.
Details of what happened are sketchy. Palestinian police said four gunmen were seen in the vicinity of where Johnston's car was found.
Palestinian Interior Minister Sayeed Sayyam said Johnston's disappearance was a "criminal act".
ALAN GRAHAM JOHNSTON
Born in Lindi, Tanzania, on 17 May 1962
Graduated with an MA in English and Politics from Dundee University, Scotland, and a diploma in Journalism Studies from the University of Wales in Cardiff
Joined the BBC World Service in 1991. He has been a correspondent in Uzbekistan, Afghanistan and Gaza
Palestinian security forces have set up checkpoints around Gaza and are searching for the correspondent.
The BBC described Johnston as a "highly experienced and respected reporter".
"It is his job to bring us day after day reports of the Palestinian predicament in the Gaza Strip," said the BBC's diplomatic correspondent, Paul Adams, himself a former Middle East reporter.
Alan Johnston was born in Tanzania and educated in Scotland. He joined the BBC World Service in 1991 and has spent eight of the last 16 years as a correspondent, including periods in Uzbekistan and Afghanistan.
He became Gaza correspondent in April 2004 and is in the last few weeks of his posting there.