The European Parliament's president has condemned the Conservatives' decision to leave the centre-right EPP group.
Hans-Gert Poettering, a German member of the EPP, called Tory leader David Cameron's decision "a serious mistake".
"The big parties should belong to a European family of parties," he said, adding that the Tories' withdrawal was "not in the interests of the UK".
The European People's Party-European Democrats group (EPP) is the biggest bloc, with 278 MEPs.
The second largest is the Party of European Socialists (PES), with 216 MEPs.
The Conservatives informed the EPP earlier this month that they intend to leave it in May.
'Regret'
They plan to form an alternative bloc after the European elections on 4 June.
The Conservatives disagree with what they see as a federalist EPP agenda.
The bloc was opposed to the UK having a referendum on the Lisbon Treaty - something the Conservatives campaigned for - and it wants closer economic integration in Europe, as well as common immigration, defence and foreign policies.
European Commission president Jose Manuel Barroso has expressed "regret" over the Tory move.
Speaking in Brussels, Mr Poettering said he was, however, glad about Prime Minister Gordon Brown's visit to the European Parliament next Tuesday.
He said it was "an expression that the UK feels the EU is important".
The Conservatives will need to find allies from at least six other countries to qualify as a new grouping and get access to EU funding.
It is thought they will try to attract MEPs from the Czech Republic and Poland among others.
MEP Graham Watson, leader of the liberal ALDE group, called Mr Cameron's move "isolationism that should have no place in British politics".
He also described it as "damaging for the UK and British business".
Mr Watson told BBC News he thought the EPP would still be the biggest bloc after the June election, despite the Tory withdrawal.
Earlier this week Mr Cameron said the new grouping would "work closely with the EPP on all sorts of areas where we agree", adding: "We will be happy neighbours rather than unhappy tenants."