by political editor Paul Francis

Ashford Conservative candidate Damian Green says the first ever debate featuring the three main party leaders could end up as "boring television" because of the strict rules and format.

Damian Green, the party’s shadow immigration minister, has had a key role in helping prepare David Cameron for tonight’s first debate by assuming the role of Gordon Brown in rehearsals.

But he said he was concerned that while the debate had the potential to fire up the campaign and would generate huge public interest, it could be less exciting than hoped.

"The viewing figures are going to be huge so it will play a significant role in the election. People are not yet hugely engaged in the campaign … but the format is very strict and that might, of course, have the potential to make for quite boring television," he said.

He also conceded that the risks for David Cameron were greater than for Gordon Brown because the Conservatives were ahead in the polls and Labour was regarded as the underdog.

"There is a risk – if you are ahead in the polls, you are there to be shot at, so yes, that might happen. But Cameron is an accomplished performer."

Mr Green is a former financial and business journalist who has presented TV programmes but said that was not why he was called upon to help Mr Cameron’s preparations.

"I was there to work through different scenarios and prepare for the types of questions that we felt might be asked."

Read Paul's blog on this and other election news and analysis here>>>

However, he said that he had not sought to adopt Mr Brown’s accent or demeanour in practice run-throughs, joking "that was beyond me."

Tonight’s debate on ITV is the first of three scheduled to take place between now and polling day. It will be devoted to domestic affairs and each of the party leaders will have the chance to make an opening and closing statement.

An audience of some 13m has been predicted for tonight’s programme.