Labour leader Ed Miliband is to accuse David Cameron of putting efforts to hold the Tory party together over Europe ahead of the national interest.
He claims the Prime Minister was forced into making his promise to hold an in/out referendum in 2017 by his own backbenchers but it would result in "four years of uncertainty" for British businesses.
Mr Miliband will insist the UK should stay inside the European Union and press for changes to "make it work better for Britain".
A series of Tory grandees including Lord Lawson and Michael Portillo have advocated withdrawal and restive Tory backbenchers hope to force a Commons vote next week in protest at the Prime Minister's failure to table legislation to pave the way for the referendum.
Downing Street has said the Prime Minister is "relaxed" about the rebel amendment and hinted he could even allow Tory ministers to support it.
But Mr Miliband will say: "I know David Cameron is a man who likes to be known for relaxing - even chillaxing - but, on this occasion, it beggars belief. He's not lying on the sofa, relaxed. He's hiding behind the sofa, too scared to confront his own MPs. 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 in the future.
"And why is the Prime Minister in this position? Because he has consistently failed to lead his party on Europe and is, instead, being pushed around by his own backbenchers. That's the only reason he changed his mind in January on his previous position on an in/out referendum. It wasn't about the national interest, it was simply about his party interest. The great irony is that it hasn't even worked because his backbenchers keep coming back for more."
The Conservatives have been shaken by the rise in support for Nigel Farage's United Kingdom Independence Party, but Mr Miliband dismissed them as lacking solutions.
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. And our national interest lies in staying in the European Union and working for the changes that will make it work better for Britain.
"It is wrong now to commit to an in/out referendum and have four years of uncertainty and a 'closed for business' sign above our country. Let me be very clear: we will always make decisions on these issues in the national interest."