Conservative leader David Cameron is facing fresh calls to abandon plans to cut links with allies in the European Parliament.
Mr Cameron had not thought through some plans, says Lord Patten
Former Tory chairman Chris Patten said the Tories had not thought through the consequences of withdrawing from the European People's Party (EPP).
The ex-European commissioner said now was not the time to find "new custard pies" to throw at the EU.
Mr Cameron says the EPP's federalist views are at odds with Tory policy.
He says his party must be consistent and shadow chancellor George Osborne this weekend said the Tories would not renege on the withdrawal pledge.
Lord Patten said Mr Cameron had made a good start to his leadership but warned that he would find it difficult to find new allies in the European Parliament.
"He made some commitments during his leadership bid and I think they weren't terribly well thought through," he told BBC News 24.
"He did it to encourage some of the Eurosceptics to vote for him which I don't think was necessary.
"The problem is that I think those who advised him to do this hadn't thought through the consequences."
His warning comes after Tory MP Quentin Davies said the Tories risked looking like "dishonest double-glazing merchants".
Mr Davies - who chairs the Conservative Group for Europe - said the move would make the party look like "gazumpers" as it would be breaking a written agreement to remain part of the group until 2009.
Shadow foreign secretary William Hague has been holding talks with other European MPs but says he has not met "oddballs and fascists".