David Cameron’s plan to withdraw the Conservatives from the main centre-right group in the European Parliament has been by criticised by business leaders who warned that it would leave Britain “on the fringes of the debate”.

The EEF manufacturers’ body told the Financial Times that “The Conservatives taking a decision that is bound to reduce our influence in Europe is not helpful to business”.

The Conservative leader has pledged to leave the centre-right European People’s Party following Thursday’s European elections in order to form a new Eurosceptic group. To gain official recognition Cameron will require at least 25 MEPs from seven different states to join the new group.

The group, which Cameron hopes will include the Czech Civic Democrats and Poland’s right-wing Law and Justice Party, is due to be unveiled next month.

Cameron’s decision is known to have alienated centre-right leaders including Angela Merkel, Nicolas Sarkozy and Sweden’s Fredrik Reinfeldt.

There is also increasing concern over his promise to renegotiate the Lisbon Treaty if elected. The Irish government is holding a second referendum this October after last summer’s no vote and voters are expected to approve the treaty. But Cameron has warned that he “will not let matters rest” even if the treaty has been ratified by all 27 member states.

In a thinly veiled criticism of Cameron last month, German Chancellor Angela Merkel told a campaign rally in Berlin: “We refuse to extend our hand to those who reject the Lisbon treaty ... and who at the same time speak of enlargement.” The treaty, which will streamline the EU’s institutions, is widely seen as essential following the accession of the Eastern European member states in 2004.

Speaking yesterday, Cameron rejected claims that renegotiating the treaty would threaten Britain’s membership of the EU.

“I don’t think [a referendum on withdrawal] is right. I don’t want us to leave the EU,” he said. “Every treaty is an effective renegotiation and if we had a Conservative government we’d be going into those sorts of negotiations with a list of powers we’d want to have returned to the UK.”

The Conservatives are also committed to unilateral withdrawal from the social chapter, which covers employment legislation. The chapter guarantees workers the right to paid holiday, to maternity or paternity leave and ensures part-time workers have the same rights as full-time employees.