Opponents of Britain’s membership of the European Union would lose a referendum if it were held now because the campaign would be run by “angry-looking grey men”, a leading Conservative Eurosceptic has said.

In a sign of the unease among anti-EU Tories that they could squander a once-in-a-generation chance to take Britain out of the union, the party’s former deputy chairman Michael Fabricant said campaigners needed to redouble their efforts to craft a positive and modern message.

The intervention by Fabricant, in an article on the Guardian’s Comment is Free site, highlights deep divisions among Conservatives after the former chancellor Kenneth Clarke condemned the “daft ambitions” of fellow party members who wanted to leave the EU. Clarke even suggested that he would like to see more colleagues defect to Ukip and urged his party not to give in to ignorance and bigotry over immigration.

Fabricant, a former whip who helped run Conservative byelection campaigns for seven years, made clear he shared Ukip’s ambition to end British membership of the EU. But he suggested opponents of the EU would repel voters if a referendum were held now.

“I do not believe that an out vote could be won right now,” he wrote. “Imagine the campaign. The polls – as they currently are – hint at a knife-edge vote. Business leaders, Labour and the Liberal Democrats, some Conservatives, the EU itself, along with heads of state and prime ministers will all warn Britain that it would be worse-off out.

“The in campaign will be run by a consensual, passionate businessperson or statesman. They will offer guarantees of reform and their campaign will be showered with money, some from the EU itself, and use slick advertising.

“The out team will be very different, with no leader who commands popular support. Before you can even make the case for Britain becoming a mid-Atlantic economic hub, freed from the shackles of Brussels diktats, the Eurosceptics will be all over the place. No clear leader and angry-looking grey men who have been arguing the toss on Europe for years will fail to impress.

“Yes, [Ukip leader] Nigel Farage is clearly the most charismatic Eurosceptic in years, but does anyone really imagine Farage being the Alex Salmond of the out campaign? Would he be persuasive enough to seduce a nation?” he asked, referring to the former Scottish National party leader who brought Scotland close to voting for independence from the UK.

Fabricant’s remarks were made as Daniel Hannan, the anti-EU Tory MEP who is a close friend of the defector to Ukip Douglas Carswell, said he feared that David Cameron was aiming to lead the campaign to keep Britain in the EU even if he failed to secure major changes in the UK’s membership terms.

In an article for the Spectator, Hannan wrote: “It is theoretically conceivable that, if he got nothing from the EU, he might pivot, sweep the country and lead Britain to a prosperous global future [outside the EU]. But I have to be realistic: he has set the renegotiation bar so low as to be able to claim victory in almost any circumstances. If he really has a contingency plan that involves seeking a free-trade-only deal with the EU, he’s the best poker player in the world.”

Fabricant and Hannan say they have no intention of following Carswell and Mark Reckless in defecting to Ukip. But their interventions highlight the fear among some anti-EU Tories that the prime minister’s planned in/out referendum in 2017 could end up killing off their dream. Some hardliners are suggesting that it would be better if the Labour leader, Ed Miliband, became prime minister to ensure that Cameron lost the Tory leadership, potentially paving the way for an anti-EU leader.

Fabricant said opponents of the EU needed to use the three years until the planned referendum to craft a sophisticated campaign.

“For those who advocate leaving the EU, the little Englander approach just won’t cut the Dijon mustard. Eurosceptics like me need to think carefully about how we craft the argument. We should avoid negative, anti-European messages and xenophobia, and instead aim to instil positive images of the sunlit uplands that will open to Britain as a prosperous global trading nation, freed from the heavy shackles of EU directives and dogma.

“My Euroscepticism is born of economics, and the campaign will need to be anchored there. But, as with the Scottish debate, the heart counts too. Eurosceptics need someone to lead the campaign who can balance the philosophical issues of the heart, with the tangible economic arguments. Until this can be done, I don’t hold out much hope for the out campaign.”