David Cameron is "hiding behind the sofa" from Tory backbenchers, Ed Miliband claims, after it emerged that the prime minister may take the unprecedented step of supporting a vote in parliament condemning his own legislative programme because it does not propose a vote on EU membership.

The Labour leader will claim that the prime minister's decision to back an amendment to the Queen's speech which criticises the absence of an in-out referendum reveals an obsession with the party's interests instead of the country's.

Miliband's speech comes as Cameron faces further pressure on Europe from senior party figures. On Friday, Boris Johnson said that withdrawal would not be "cataclysmic" for British jobs, as Europhiles claim. The mayor of London was speaking after Tory grandees Lord Lawson and Michael Portillo this week advocated withdrawal from the EU and Tory backbenchers sought a referendum amendment to the Queen's speech. Internal Conservative rows over Europe have escalated following the strong showing of Ukip in the local elections this month.

Miliband's speech, to the annual conference of the Blairite thinktank Progress, will also be seen as an attempt to stamp his authority on his own party after YouGov polling showed that his personal polling has hardly improved over the last year.

Miliband will say: "We have the extraordinary spectacle of a government that says it is relaxed about its own MPs voting against its own Queen's speech.

"He [Cameron] is not lying on the sofa relaxed. He's hiding behind the sofa, too scared to confront his own MPs. He is flailing around, directionless, unable to show the leadership the country needs."

The reason for this weakness, Miliband will say, is that Cameron has allowed backbench MPs to push him around. "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."

Conservative MPs have engineered a vote on an EU referendum by tabling an amendment to the Queen's speech. The move has the support of longstanding Eurosceptics such as John Redwood, Peter Bone, Edward Leigh, Bernard Jenkin and David Davis. It is likely that the Speaker will accept the amendment, with a vote in the Commons on Tuesday or Wednesday.

It is unlikely the amendment will be passed, as the Lib Dems will oppose it along with the vast majority of Labour MPs, but significant minority support within the Tories would be seized upon as an indication that Cameron is being dictated to by his party's backbenches.

On Friday, the prime minister's spokesman refused to rule out supporting the amendment. "The amendment's only recently been tabled, so the PM would want to carefully consider it.

"The key point here is that he has always made it clear that he will look at all ways of strengthening his commitment to an in/out referendum in the next parliament.

"He is happy to look at all ways of strengthening his commitment to an in/out referendum. I'm not going to speculate about what may happen on a vote that hasn't actually been called yet," he said.

The Downing Street spokesman declined to say today whether a whip would be imposed on Conservative MPs.

The prime minister is even considering voting for the change to the government's programme, though this is highly unlikely because he is due to be in the US next week. At present Cameron wants to hold the referendum after he has completed negotiations with his EU partners after the 2015 general election.

The rebel amendment was signed by 32 MPs, including 30 Conservatives and two from Labour – Kelvin Hopkins and John Cryer.

Johnson told BBC Radio 4's Today programme: "We should be prepared to pull out. That goes without saying. You can't go into a negotiation like that without being willing to walk away."

He insisted that withdrawal would not be as damaging to the UK economy as pro-Europeans claim. "I don't think it would be anything like as cataclysmic as is being sometimes pretended," he said.

Labour is expected to impose a whip if the amendment comes before parliament, which will provoke clashes with a number of senior backbench MPs. Labour insiders said that Miliband, in his speech, is emphasising that his party is sticking to long-held pro-European principles while the Tories shift according to their party's whims.

"Our national interest lies in staying in the European Union and working for the changes that will make it work better for Britain," Miliband will say.