Labour is expected to accuse the Prime Minister of "sleepwalking" towards British exit from the European Union later today.
"The real tragedy of tomorrow's speech is that David Cameron's party won't let him address the undoubted need for change in the EU in a sensible way," Shadow Foreign Secretary Douglas Alexander will say.
"We have a Prime Minister who simply cannot reconcile the demands of his party with the needs of the country.
"The gap between the minimum the Tories will demand and the maximum our European partners can accept remains unbridgeable.
"For many in his party, getting David Cameron to commit now to an in/out referendum is not about securing consent. It is about securing exit.
"And we will have a British Prime Minister sleepwalking towards exit, knowing he is letting down the national interest, but too weak to do anything about it."