The rock star - a close ally of Gordon Brown on tackling poverty and repression in Africa - will join Mr Davis on the campaign trail.

He said: "What is the point of Britain without its freedoms? What terrorises the terrorists is our civilisation.

"I am delighted to support this campaign and I will be speaking on it in Hull tomorrow."

The backing, from someone he so regularly shares a platform with on development issues, could prove embarrassing for the Prime Minister.

Mr Brown was forced to deny again today that he had resorted to "grubby deals" to squeeze the extension to pre-charge detention through the Commons last month.

Mr Davis quit as MP for Haltemprice and Howden last month so he could fight a by-election on the issue, accusing the Government of "trampling" on essential freedoms.

However, Labour has refused to field a candidate, and ministers have been doing their best to ignore the contest, which culminates on July 10.

Mr Davis said he was looking forward to the event with the former Boomtown Rats star.

"I am delighted that Bob Geldof is supporting our campaign, which stretches across all parties and is vital to the freedom of our country," he added.