A "£9bn hole" in the British defence budget means Britain should drastically restructure its armed forces, the authors of an independent report have claimed.

A "£9bn hole" in the British defence budget means Britain should drastically restructure its armed forces, the authors of an independent report have claimed.

The report, from a commission chaired by former defence secretary Lord Robertson of Port Ellen and former Liberal Democrat leader Lord Ashdown of Norton-sub-Hamdon, urges ministers to consider whether the Trident submarine-based system is the most cost-effective way of maintaining Britain's "minimum" deterrent capability.

However, Downing Street said it did not recognise the £9bn figure.

Both Labour and the Conservative opposition are committed to a £20bn Trident replacement programme but the report for the IPPR think-tank suggests that while Britain still needs a minimum deterrent, the decision should be revisited as part of a strategic review of security going beyond just defence.

One option, the report recommends, could be a further service extension of the ageing Vanguard class submarines, which carry the Trident missiles, beyond the additional five years to 2024 already planned. Defence experts counter that extending the lifetime of the current submarine hulls, which withstand incredible underwater pressure during their service, is not a viable option.

Cutting the replacement fleet size from four boats to three would leave the UK without a permanent deterrent because of the servicing and maintenance cycle of the nuclear submarines.

The recommendations on Trident form one of a raft of proposals covering the whole spectrum of Britain's national security. The report calls for a review of £24bn of existing defence equipment projects with a view to making cuts, with the new aircraft carriers, the joint strike fighter, the Type 45 destroyers and the Astute class submarines all "in the frame".

Lord Ashdown, who co-chaired the IPPR commission, said he believed there was now a £9bn "black hole" in the MoD's annual budget of £36bn. He said that overspends in Afghanistan and Iraq, together with commitments to outdated equipment, ships and strategies cost the UK dear.

At a command level the report called for the creation of a joint civilian-military stabilisation and reconstruction taskforce and far better international military co-ordination.

The commission also called for major innovations in defence policy to adapt to the "post-9/11 and post-recession world", with investment in cyber-warfare, command and control. It also recommended that there should also be strengthening of the special forces in order to deal with a Mumbai-style terror attack in the UK.

More broadly, the commission called for a single, cross-departmental security budget, and a new national security council to deliver strategic thinking on the country's resilience.

Lord Robertson said that in the current climate, greater European co-operation in Nato was essential, although he did not back the idea of a European army.

The report will carry some influence in Whitehall, where strategic defence thinking is already being reviewed. Defence Minister Bill Rammell said it was wrong to suggest that alternatives to the submarine-based Trident system may be more affordable.

He said: "We don't put forward proposals to invest in equipment unless we believe it is necessary. We remain committed to the policy we set out two years ago on Trident. We keep it under constant review and that is not very much different at all to what this report is saying."

He added: "We remain committed to working towards a world free of nuclear weapons. We are the most forward-leaning nuclear state in terms of disarmament - we have reduced the explosive capability of our nuclear arsenal by 75% over the last 10 years.

"But when we look at the risks moving forward over the coming decades, we don't believe at the moment it would be safe to fail to make decisions now which would effectively commit us to unilateral disarmament in the future, regardless of the circumstances."