David Cameron is preparing to threaten to back Britain quitting the European Union if it fails to make major concessions over returning powers.
The Prime Minister has always refused to say he would campaign to sever ties if he failed to achieve his promised reforms, but is now poised to issue the warning, according to The Times.
Mr Cameron has pledged that a Conservative government would hold a referendum on EU membership by the end of 2017 after renegotiating London's relationship with Brussels.
His efforts to pave the way for a new deal were dealt a blow earlier in the summer when he lost the battle to stop arch-federalist Jean-Claude Juncker becoming the next European Commission president.
The PM wants to Britain to be freed from the commitment to an ''ever closer union'' as well as a shift to powers flowing away from Brussels rather than to it.
Among the other reforms he wants to secure are a reduction in red tape for businesses and the right to free movement to take up work, not free benefits.
But critics warn that a significant overhaul is unlikely and he faces a major battle to win over Mr Juncker after publicly denouncing his candidacy.
No timescale has been set for outlining the beefed up stance but the Conservative party's autumn conference is being considered, according to The Times.
A source told the newspaper: "The shift was almost made public at the time Juncker was appointed, but that timing might have looked like petulance."