Police and security forces have stepped up their anti-terrorism measures in an attempt to thwart any bid by Irish terrorists to disrupt the closing stages of the election campaign.

Metropolitan police officers have been put on alert for a large vehicle bomb similar to those used in the past, including one that exploded outside the City's Baltic Exchange within 24 hours of the 1992 general election.

Scotland Yard has asked the public to report anything suspicious to its anti-terrorist hotline on 0800 789321.