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.
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.
Faced with a split party, Cameron could learn from Harold Wilson and suspend collective cabinet responsibility for the referendum campaign.
It is odd to speak of an EU referendum as inevitable when few believe the Conservatives will win a majority at the next election.
The EU is an asset for Britain, not a hindrance. Unlike Tory MPs, our businesses haven't forgotten that.
The Mayor's demand for an EU referendum before 2015 means it will be harder for Cameron to impress.
Regional parties mitigate the threat of a single populist party winning national support.