Businessmen have appealed to David Cameron to drop his plan to pull the Conservative Party out of the main centre-right political group in Europe.
Some British companies fear that Tory MEPs would lose influence in the European Parliament if they break away from the European People's Party (EPP-ED). They say this would limit the ability of firms to secure amendments to legislation affecting them by the European Commission, which has to be approved by MEPs.
Bosses are also worried that a future Tory government would have less clout in the EU if it was seen to be on the margins rather than allied to its centre-right partners.
The Confederation of British Industry is keeping out of the Tories' internal dispute, but yesterday Business for New Europe, a recently formed group of pro-Europeans, which has 20 senior businessmen on its advisory council, urged caution.
Zaki Cooper, the group's director, said: "Obviously, the business community is interested in what happens here. From its point of view, the question is how UK business can be most effectively promoted within the European Parliament.
"It would take some convincing that withdrawing from a mainstream grouping would achieve this."
Mr Cameron promised to withdraw Tory MEPs out of the EPP-ED group during last year's Tory leadership election. But attempts to form a new group of like-minded reformers have foundered.
A Tory spokesman acknowledged that there were mixed views in the business community on the issue, as there were over joining the single currency. But he said: "We believe a majority of British companies would support us because they want to see reform in Europe and less regulation from Brussels."
However, one Tory frontbencher said: "Some of our biggest firms are getting very steamed up about this and are lobbying us hard. We don't want to get on the wrong side of business and we should drop it."
Yesterday Mr Cameron stayed away from the EPP-ED's traditional meeting ahead of the two-day summit of EU leaders in Brussels. His office said he had a busy schedule, but Hans-Gert Pöttering, leader of the group, said: "I would have liked to see David Cameron here but if someone doesn't want to have a dialogue it is his decision."
Geoff Hoon, the Minister for Europe, said: "David Cameron's continuing absence from EPP-ED meetings confirms how marginalised and isolated the Tories have become in Europe. Not only has David Cameron's pledge to quit the EPP-ED left the Tories marginalised and isolated, it has confirmed that the same old Tory divisions on Europe remain."
Centre-right politicians including Angela Merkel, the German Chancellor, and Nicolas Sarkozy, the French Interior Minister, have criticised the Tory pledge to quit the group.
Some parties within it have threatened to stop working bilaterally with Mr Cameron if he goes ahead.