Senior opposition politicians yesterday questioned why Britain is spending £6.4bn on its strategic nuclear deterrent while basic combat training for troops is being cancelled on cost grounds.

They also called for a comprehensive defence review to prioritise the country's military needs and tailor funding to match the threats.

The LibDems said 36 exercises had been cancelled since 2003 to save money, including 18 in the past two years as fighting in Iraq and Afghanistan intensified to levels not seen since the Korean War.

Nick Harvey, the party's defence spokesman, said: "This is an issue that goes beyond our spending on the nuclear deterrent.

"We need a complete rethink on how the entire defence budget is spent.

"Gordon Brown has become very fond of boasting about how much money is spent on defence, but the sad fact is that basics like training and accommodation are still being left desperately underfunded."

The SNP meanwhile revealed that the Ministry of Defence has committed more than 5% of its entire budget over the next three years to maintain the Royal Navy's four Trident missile submarines and the Atomic Weapons Establishment warhead plant at Aldermaston.

According to the MoD's own figures, revealed in a Parliamentary written answer to Alex Salmond, the SNP leader, keeping the nuclear arsenal credible will cost £3bn between now and 2009, and another £3.4bn from then until 2011.

More than £2.6bn is scheduled to go on Aldermaston, where a new super-laser and powerful computers have been installed to help design the next generation of nuclear warheads.

A total of £30m has also been earmarked "to examine the optimum life of the UK's existing nuclear warhead stockpile and assess the range of replacement options" under a programme run by the Warhead Pre-Concept Working Group.

This follows the government's decision to replace the four existing Vanguard-class missile boats from the mid-2020s and equip them with new "reliable" warheads already believed to be under development in co-operation with the US.

The MoD categorically denied that any soldier bound for frontline service had been deprived of essential training for deployment to war zones, but conceded that 127 "training events" had been cancelled or postponed since 2005.

Of these, more than 30 had to be scrapped as a result of "operational commitments"- mainly because the troops due to take part were either in Helmand or Basra or had just returned from tours of duty.

"All personnel undergo three months of pre-deployment training before going to either Iraq or Afghanistan.

"No-one is being deprived of essential preparation for role. Scheduled exercises have been cancelled for a variety of reasons including operational tempo," a spokesman said.

Each of the Royal Navy's four Vanguard submarine carries a maximum of 48 warheads on 16 Trident missiles. The UK's total nuclear stockpile is understood to be fewer than 160 warheads.

A number of former military commanders have spoken out in the past year on the fact that training cutbacks and overstretch of ever-scarcer manpower is eroding the UK's future ability to fight a war against any technologically-competent enemy.