At least four senior figures in the City of London, including the Lord Mayor, alleged they had been subjected to a campaign of "intimidation and dirty tricks" after they made clear they believed it would be in Britain's interests to join the euro.
According to City sources, the warnings by prominent Eurosceptic businessmen amounted to a threat. Lord Levene, the Lord Mayor, and Judith Mayhew, political leader of the City of London Corporation, have both been warned their future careers in the City could suffer if they publicly backed the single currency.
Ms Mayhew, who is understood to have informed the police, saw the warnings as a veiled threat to remove her from her post as chairman of the corporation's policy and resources committee. She is two years into a planned five-year term but comes up for re-election each year.
Ms Mayhew, a City lawyer, dropped plans to serve on the board of the pro-euro Britain in Europe campaign after criticism by Eurosceptics earlier this year and has since remained neutral on the issue.
Britain in Europe, to be launched next month by 60 businessmen, claimed last night that similar threats had been issued to two of its leaders. It said Lord Marshall, the British Airways chairman, was told that some shareholders would sell their holdings in the airline if he fronted the campaign, and that Lord Sharman, managing partner of KPMG International, would see his company lose consultancy contracts if he joined the pro- euro drive.
One board member of Britain in Europe told The Independent: "These are the sort of intimidatory tactics we have had to get used to. But we will not give in to such threats."
There is no suggestion that the threats were orchestrated by Business for Sterling, the group leading the fight against the single currency. It is holding a conference today - Advantage Britain - to be attended by about 500 businessmen.
Nick Herbert, the group's chief executive, said last night: "We would have nothing to do with any such threats. I don't believe it. Britain in Europe is showing all the signs of a campaign in crisis. It sounds like a feeble cry." However, he added: "It is true that there is bound to be shareholder disquiet if the chairman of a big public company campaigns in favour of the euro when a majority of the public are against it."