A high-profile poster campaign in the UK was launched this week

Delegates voted unanimously to support the campaign to free him at their annual conference in Birmingham.

Jon Williams, editor of BBC World News, said: "Alan stayed when everyone else left because he believed that the story of Gaza must be told."

The BBC held a day of action on Thursday to highlight his plight.

'Calmness and courage'

Mr Johnston is presumed to have been abducted when he disappeared on 12 March after leaving his Gaza office.

Mr Williams told a packed hall that, as every day passed, concern for the reporter's physical and mental wellbeing was growing.

But he paid tribute to his calmness and courage and said he was an example of the need to be able to report freely.

Mr Johnston was due to finish his posting in March

General Secretary Jeremy Dear read a message from the Palestinian Journalists' Syndicate (PJS) detailing solidarity marches and other demonstrations held in Gaza in an attempt to free the BBC correspondent.

An online petition calling for the release Mr Johnston has gathered more than 30,000 signatures.

The 44-year-old 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 has lived and worked in Gaza for three years and was the only Western reporter permanently based in the often violent and lawless territory.

His posting in Gaza had been due to end in late March.