ED Miliband today brands the Prime Minister "weak and panicked" and unable to stand up for the national interest because of the continuing Tory rebellion on Europe.

The Labour leader will use a keynote speech to the Progress annual conference in London to launch a scathing attack on David Cameron's premiership, which is coming under increasing pressure over the possibility of a referendum on the UK's European Union membership.

A senior Opposition source claimed it was Mr Miliband who was now acting like a prime minister while the Conservative leader was "chasing every will o' the wisp on the backbenchs" and had "completely lost the plot".

There is expected to be a vote next week on a Tory rebel amendment decrying the fact there was nothing in the Queen's Speech about an EU referendum. Yesterday it had already been signed by 30 Conservatives, including David Davis, the former Tory leadership challenger, and former ministers John Redwood and Cheryl Gillan.

While No 10 insisted the Prime Minister was relaxed about the vote and welcomed the "spotlight being put back on his commitment" to hold an in-out referendum in 2017, the pressure on him intensified after yet another intervention from a senior Conservative.

Boris Johnson, the London Mayor, said the PM was absolutely right to seek a renegotiation of Britain's terms of membership of the EU. However, the would-be premier also declared: "We should be prepared to pull out. I don't think it would be anything like as cataclysmic as is being sometimes pretended.

"I don't think we would lose millions of jobs. The economic benefits and disbenefits are now much more balanced."

In his speech, Mr Miliband will mock the PM, saying it is extraordinary that a Government is relaxed about its own MPs rebelling against its Queen's Speech. He will say his opponent is not lying on the sofa relaxed, but "hiding behind it, too scared to confront his own MPs".

The Labour leader will add: "It's not chillaxing. It is weak and panicked. He's flailing around, directionless, unable to show the leadership the country needs because on this, as on so many issues, he has no answers to the challenges facing Britain."

Mr Miliband will accuse the PM of being "pushed around" by his eurosceptic backbenchers.

Referring to the leader of the UK independence party, Nigel Farage, he will say: "David Cameron may try to out-Farage Farage on Britain's membership of the European Union, but we will always stand up for the national interest. Our national interest lies in staying in the European Union and working for the changes that will make it work better for Britain."