Nicolas Sarkozy, the French president, is to deliver a firm warning to David Cameron that the Conservatives risk forfeiting vital French co-operation on energy, defence and the economy if they refuse to engage over the future of Europe.

In an attempt to lure the Conservatives into a friendlier stance, Sarkozy may be willing to offer concessions over the future of the European defence agency, seen by the party as the incubator for a future European defence force.

Sarkozy is due to meet Cameron on Friday after a working lunch with Gordon Brown in Downing Street. Sarkozy has developed close relations with Brown.

The EDA was set up in 2004 to develop European military capability and armament co-operation.

The Conservatives have threatened to withdraw from the body and the French may be willing to see it disbanded, or radically reformed, as part of a move to shift the focus to greater bilateral co-operation between France and Britain.

The greater French strategic interest is in ensuring greater co-operation between the two countries on defence procurement to ease the pressures on both countries' budgets.

In a speech today William Hague, the shadow foreign secretary, will hint at a willingness to work with the French when he states that a key Conservative foreign policy objective will be to "retain the ability to project power on a strategic level when working alongside the United States or France, with speed, precision, safety and effect".

The French have been lavishing attention on the Eurosceptic shadow defence secretary Liam Fox, who raised the possibility of a Conservative UK pulling out of the EDA in a recent speech in Paris. But Fox also believes Anglo-French defence co-operation is in both countries' mutual interest, and should form an important element of the UK strategic defence review. Sarkozy is anxious to see Anglo-French co-operation prosper whoever is elected in the UK and is working hard to persuade Cameron to avoid confrontation over the EU powers.

Conservative-French relations hit a low last November when Pierre Lellouche, France's Europe minister, described as "pathetic" the Tories' EU plans. He warned that they would not succeed "for a minute".

Sarkozy is not expecting any early movement from Cameron if he is elected prime minister on his party's decision to leave the European People's party, the centre-right grouping in the European parliament. But the French are hoping for signs that Cameron will at least set out in private what concessions he wants in his negotiations with the EU. The Tory leader has said it may take five years to repatriate powers.

Cameron will also be looking for assurances from Sarkozy that the French will not back German plans for further treaty changes to bring about a European monetary fund, an idea floated this week by the German chancellor Angela Merkel.

Sarkozy wants British agreement on greater energy co-operation, a stronger price for carbon in Europe, defence co-operation and measures to prevent Europe slipping back into recession. The French owened ED has made massive investment in the UK energy market, and is still not yet clear what the Conservative energy policy will be on nuclear and carbon subsidies.

In a bid to understand the Conservative thinking, French ministers have staged bilaterals with senior shadow cabinet members including Hague, George Osborne,Chris Grayling and Michael Gove.Last night Chris Bryant, the Europe minister, attacked the Conservatives over their position on Europe. He said: "The Tories want to avoid talking about Europe, they don't want any questions before the election about Europe."