The two former cabinet ministers will throw their full weight behind the "Britain in Europe" group once elections for the European Parliament have been completed this summer.
The new organisation, which holds its first meeting in the City of London today, aims to persuade the public to vote "yes" in any forthcoming referendum on the euro.
The cross-party campaign will bring together MPs, businessmen and trade unionists to launch a nationwide publicity drive to counter the plethora of Euro-sceptic groups that have sprung up.
Initially, more than 100,000 leaflets will be delivered in 125 towns and cities across the UK, followed by a pounds 2m-a-year media blitz including newspaper advertisements and billboard posters.
Lord Marshall, the former CBI president and current British Airways chairman, formally launched the group yesterday when he confirmed the membership of its executive board.
Lord Hollick, the Labour peer and chief executive of United News and Media, will join Menzies Campbell, the Liberal Democrats' foreign affairs spokesman, and Ken Jackson, general secretary of the AEEU, on the board.
Lord Howe of Aberavon, the former Tory chancellor, is also on the board in the capacity of an "observer" and is so far the most senior Conservative involved in the campaign.
However The Independent understands that Mr Clarke and Mr Heseltine will sign up after June's European elections and join Britain in Europe's full council. Both men have decided to steer clear of explicit backing for the group before June in order to reduce Tory divisions ahead of the elections to the European Parliament, but are certain to join after then.
Their involvement became even more likely yesterday after Mr Hague softened significantly his line on dissent from pro-European colleagues, stating that he wanted his party to remain a broad church. "It doesn't stop people being a Conservative just because they disagree on one policy. People are allowed to be Conservative without agreeing to 100 per cent of the policies," he told BBC's On the Record programme.
"I do not want to see MPs driven out of the Conservative Party or excluded from the Conservative Party. Those people are Conservatives."
Britain in Europe will be the main challenger to the Democracy Movement - a Euro-sceptic campaign funded by the Yorkshire businessman Paul Sykes.
In an attempt to outmanoeuvre the better funded "no" campaign, the pro- Europeans are to place features in women's magazines to list the benefits of the euro, explaining how a single currency would make everyday life more efficient. "Women are the driving forces in many families because they are often still responsible for the household and shopping," said a source close to the campaign.
"They have enormous influence over their families and if we can convince them, we are nearly there. Their euro-scepticism is often a gut feeling and we have to present them with facts."
Lord Marshall, a cross-bench peer, said yesterday: "We believe the euro will secure the best future for a modern Britain in a modern Europe: prosperous at home, strong in the European Union, proud in the world. This campaign aims to inform the British people about the single currency and convince them of the advantages to Britain of joining."
Other members of the Britain in Europe board announced yesterday are Colin Sharman, chief executive of KPMG International, Niall Fitzgerald, chairman of Unilever, and Judith Mayhew, chairman of the policy and resources committee of the Corporation of London.Reuse content