Nothing has been heard from Mr Johnston for three weeks

David Dimbleby, Sir David Frost, Jon Snow and Christiane Amanpour of CNN are among hundreds of people backing a full-page advert in Monday's Guardian.

The move comes three weeks to the day that Mr Johnston, 44, disappeared.

Palestinian journalists have begun a three-day strike urging the government to do more to secure his release.

HAVE YOUR SAY Alan should be released and be able to continue to do what he does best Ahmed Duaij, Kuwait Your messages to Alan

"It could have been any of us," Channel 4 News presenter Jon Snow told the BBC. "We have to stand together against this."

The BBC has had no contact since 12 March with Mr Johnston, who has lived and worked in Gaza for the past three years.

He is widely believed to have been kidnapped, but no demands have been made public.

Boycott

The Guardian newspaper advert, signed by dozens of Mr Johnston's friends and colleagues, calls on everyone with influence to increase their efforts "to ensure that Alan is freed quickly and unharmed".

Other signatories include:

• BBC Director-General, Mark Thompson
• Director of channels at Channel 4, Kevin Lygo
• Editors of several London newspapers
• Wadah Khanfar, editor-in-chief of Arabic TV network al-Jazeera

Meanwhile in Gaza, the head of the Palestinian Journalists' Union, Naim Tubasi, told the BBC it was necessary to boycott Palestinian government proceedings because it was doing little to help secure the journalist's release.

During the boycott, Mr Tubasi said no Palestinian media outlet would cover stories on the activities of the presidency or the new unity government. He said the protests would continue until Mr Johnston was freed.

Palestinian journalists plan to step up action for Alan Johnston

Simultaneous demonstrations are also being staged on Monday in Ramallah and Gaza City.

There will also be demonstrations outside BBC offices in the UK and bureaux around the world.

Intensive international efforts have been going on to secure his safe return.

These include appeals from the Arab League and the European Union and non-government groups such as Amnesty International and Reporters Without Frontiers.

On Monday, the International Press Institute, a media watchdog, expressed "grave concern" about Alan Johnston's fate.

IPI director Johann Fritz called on anyone with influence in Gaza to intensify their efforts to ensure his freedom.

"His work reporting fairly and accurately from Gaza has been widely praised. He has shed light on one of the world's most demanding stories - in keeping with the highest standards of the BBC," Mr Fritz said in a statement.

Senior BBC colleagues of Mr Johnston have appealed to Palestinian leaders including President Mahmoud Abbas and Palestinian Prime Minister Ismail Haniya.

Both have said everything possible is being done to free Mr Johnston.

The BBC describes him as a highly experienced and respected reporter.

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.