The shadow foreign secretary says that only Labour can "give voice to the national interest" in an address at Chatham House.
Prime Minister will remain in London tomorrow to chair Cobra meetings, rather than delivering long-delayed speech in the Netherlands on Britain's EU membership.
Labour leader says he would not reverse measure previously denounced by his party as a "dog's breakfast" and a "political gesture".
Conservative MP Robert Buckland says it is time for the UK to engage with the EU constructively.
There is no evidence that any EU member is willing to allow Britain to pick and choose which laws it obeys.
If Ken Clarke is free to put the europhile case, eurosceptics will want to be able to argue for withdrawal.
Lib Dem leader struggles to explain away election leaflet that called for a "real referendum" on EU membership.
Downing Street confirms a date for the PM's long-delayed speech on "the future of the EU and the UK's relationship with it".
Faced with a split party, Cameron could learn from Harold Wilson and suspend collective cabinet responsibility for the referendum campaign.