Britain's main political parties have joined forces to oppose Jean-Claude Juncker as the next president of the European commission. Labour announced on Monday that its MEPs would vote against the former prime minister of Luxembourg.

After intense Tory pressure on Labour to clarify its position, the party said on Monday that its MEPs would seek to block the veteran EU fixer on the grounds that he would make reform more difficult.

Labour had initially declined to say how its MEPs would vote on the grounds that the European council – the body comprising the EU's 28 leaders – had yet to exercise its powers to propose a candidate to lead the European commission. Such a candidate would then have to be approved by a majority of MEPs.

But as David Cameron intensifies his campaign against Juncker at a meeting with Angela Merkel and his Swedish and Dutch counterparts in Stockholm, Labour decided to make clear that it will side with the Tories and Liberal Democrats in opposing Luxembourg's former prime minister.

A Labour spokesperson said: "The nominee for European commission president is ultimately a decision for the European council, including David Cameron.

"Labour will not support Jean-Claude Juncker as a candidate … should Mr Juncker be put before the European parliament, Labour MEPs would vote against him.

"The message from the European elections was clear – that we need reform in Europe. We need reform so we can promote jobs and growth. Mr Juncker's record shows he would make these reforms more difficult."

The statement will neutralise Tory attacks on the Labour leader, Ed Miliband, for failing to speak out against an arch federalist. But the Labour move will make little practical difference. If Juncker, who is the candidate of the centre-right EPP grouping in the European parliament, succeeds in winning the support of the European council he would probably secure enough support in the parliament.

Cameron is focusing his efforts on building up a "blocking minority" in the council to prevent Juncker's nomination. He will call on Merkel as well as the Swedish and Dutch prime ministers – Fredrik Reinfeldt and Mark Rutte – to abide by the Lisbon treaty which says the council has the sole power to nominate a candidate taking account of the European elections.

In the view of No 10 a candidate from the EPP should be the next European commission president because the grouping won May's European parliamentary elections. However, Cameron believes that a wider array of candidates, possibly including Reinfeldt, should be considered. Britain believes that the selection of Juncker as the EPP candidate at a special conference in March in Dublin was part of a crude move by the European parliament to stamp its mark on the process.

After speaking at the International Festival for Business in Liverpool on Monday, Cameron outlined what he would say to Merkel and other European counterparts in Sweden about Juncker. He said: "The point I will be making is what matters is that we have people running this organisation that understand the need for change and reform. The programme is as important as the people. What is the work programme for the commission and for the European council in the next four years?"

He said important questions were whether Europe would have a digital single market and an energy single market and would Europe become more energy resilient.

"My reaction to the fact that in Britain an anti-Europe party topped the poll [and the Front National did the same in France] is not to bury my head in the sand. We need to make sure we are engaging with the public and changing some of the things that Europe does. But it's about progress most of all."