Anglo-French defence co-operation on everything from nuclear warheads to transport aircraft, helicopters and aircraft carriers would bolster the future of a cash-strapped western military alliance, Nato's secretary general has said.

Anders Fogh Rasmussen threw his support behind suggestions that London and Paris were exploring a radical departure in defence policy, in seeking a deal under which French engineers maintain British nuclear warheads. David Cameron and Nicolas Sarkozy will hold a summit in three weeks at a UK naval base with military collaboration high on the agenda.

A breakthrough between the UK and France could provide the model for sustaining Nato at a time of financial crisis and slashed defence budgets, Rasmussen told a group of journalists, including the Guardian, in Brussels. "It's very much what I said is the way forward, to identify assets and capabilities that could be shared," he said. Paris and London should pool resources on "laboratories, shared services, and maintenance", he added, in reference to proposals that Britain's 160 nuclear warheads should be transported to France for servicing by French atomic scientists and engineers.

Rasmussen also warned that looming British defence budget cuts must not impair the UK's ability to be a military force beyond its borders. He said: "I feel confident the British government will make sure it primarily cuts fat while also building up muscle."

With the defence secretary, Liam Fox, fighting chancellor George Osborne over the scale of defence cuts that could reach up to 20% over four years, Rasmussen said the slashed budget must not affect Britain's "deployable assets, transport and hi-tech capabilities" and added: "It's an essential part of British security policy to continue to defend its interests beyond the UK's borders." Earlier he warned that western prosperity would be imperilled if defence cuts went too far, saying: "There is a point when you're no longer cutting fat, you're cutting into the muscle and then into the bone."

Cameron and President Sarkozy are to meet next month for a summit amid growing speculation about pioneering new military pacts. In recent weeks ideas about sharing nuclear submarine patrols and aircraft carriers have been floated, only to be promptly dismissed. The mutual nuclear warhead maintenance is the latest signal of attempts to seal an agreement.

"There's a pattern," said Tomas Valasek, a defence analyst at the Centre for European Reform. "France and the UK have the same problems, both with nuclear forces and the full spectrum of military capability. They face the dilemma of dramatic spending cuts while trying to retain European superpower status. If they don't want to reduce capabilities they need to seek savings through collaboration. That's the conclusion they're coming to."

A paper last week by a leading French security analyst for the Royal United Services Institute called for far-reaching decisions on defence collaboration if both countries were serious about the "long-term strategic survival of their militaries". "The negative trends of economic austerity, defence inflation and a demanding strategic environment are rapidly converging into a downward spiral that, if nothing is done, will prove the end of the defence game for the two middle powers in Europe," wrote Etienne de Durand. "British and French military capabilities will rapidly diminish beyond repair."

Valasek said that while the notion of Anglo-French nuclear co-operation might raise hackles in Washington, the Americans were more worried about declining military capacities "among some of its best allies", notably Britain.

"[Liam] Fox got an earful from the Pentagon in Washington recently."

Before Cameron and Sarkozy stage their summit, Osborne's public spending review and the MoD's strategic defence review are to be unveiled, revealing the potential scope of budget cuts.

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