Eddie Izzard has urged voters to punish David Cameron for "slagging off" Britain.
In Labour's latest party political broadcast, the comedian insisted the country was "bloody brilliant" rather than "broken", as the Tories have claimed.
Izzard, who last year completed 43 marathons in 51 days for Sport Relief, is seen against a plain grey background, speaking directly to the camera.
"Last year I ran round Britain and you know what? Britain is bloody brilliant," he said. "I take great offence that the Tories are slagging off Britain saying it's broken.
"Up and down the country I met people who were working hard, creating projects, looking out for people.
"If the Tories bothered to get on their bikes and go around the country they might actually find the real Britain."
Izzard conceded Labour was not perfect but he "truly believed" the party's "heart was in the right place".
He also warned voters the Tories would be using "shedloads of money" from wealthy backers such as Lord Ashcroft during the campaign to make themselves look good.
Meanwhile, a group of senior public figures urged politicians of all parties not to talk Britain down as they fight for votes.
The warning came in an open letter signed by business and cultural leaders, including prominent Labour supporters such as EMI boss Charles Allen, actor Lord Richard Attenborough and designer Sir Terence Conran, alongside Muslim Council of Britain secretary general Muhammed Abdul Bari and new EasyJet chief Carolyn McCall.
"Our purpose in writing is to ask all of you to ensure that the forthcoming election campaign avoids damaging the reputation of Britain," the missive said.
"Investor confidence, tourism and our own self-esteem can all be destroyed by painting a distorted picture.
"We think it will be vital to British interests that this election campaign avoids damaging the reputation of the country.
"Our country has much to be proud of - the product of all parties work - so we look to all party leaders to ensure that Britain's reputation as a country is strengthened not weakened through the campaign, whoever is the victor on Election Day."