David Cameron is facing growing pressure from within the Tory ranks to stage a referendum on Britain's future in the EU.
Mark Pritchard, the secretary of the influential Tory backbench 1922, has warned that Conservative MPs "can no longer be taken for granted" on the issue.
Writing in The Daily Telegraph, he described membership of the EU as a "burdensome yolk, disfiguring Britain's independence".
His intervention comes amid growing frustration among Tory eurosceptics at the failure so far of the Government to repatriate powers from Europe in the face of opposition from their Liberal Democrat coalition partners.
Last week 120 Conservative backbenchers gathered at a private meeting in Westminster to voice their impatience at the lack of progress on the issue - a theme certain to surface again at the party's annual conference next month in Manchester.
Lib Dem Treasury Chief Secretary Danny Alexander risked further inflaming opinion on the Tory benches, telling his party's conference in Birmingham that the "anti-Europeans" were the "enemies of growth".
In his article, Mr Pritchard wrote: "Conservative MPs will not continue to write blank cheques for workers in Lisbon while people in London and Leicester are joining the dole queue. For many Britons, the EU has already become a kind of occupying force."