Cameron's opposition Conservatives should have the issue of change as all their own, but the breakthrough by Clegg means he now occupies some of the same political ground, and he threatens the Conservatives enough to hold them back from gaining the support they need to form a majority government.

The loser of Thursday's TV debate was certainly Prime Minister Gordon Brown, who put in a good performance, but who failed to impress viewers. It also came a day after he described a pensioner he met while campaigning as a "a bigot," an unguarded comment which pushed his already strained re-election campaign off the rails.

Friday saw more calamity for Labour as the unveiling of a new poster campaign attended by Brown and many of the senior figures in the party was overshadowed by a car crash that occurred just meters away from the unveiling ceremony.

Labour did have one unusual tactic up their sleeve, however. They finally brought into play former Prime Minister Tony Blair, who has so far played only one brief part in the election campaign at the very beginning.

Blair, currently a peace envoy in the Middle East, said his appearance in the campaign had been delayed by the grounding of aircraft across Europe a fortnight ago by the dust from an Icelandic volcano.

He offered support for the embattled Brown, "I believe Labour has every chance of succeeding," he said. "When you get into the final days people will really focus their minds on who's got the best ideas for the future."

He dismissed the effects of the TV debates, saying they were part of the "ups and downs" of campaigning.