The MoD took the unusual step of issuing the assurance following reports that Britain was preparing to buy up to 50 Tomahawk cruise missiles from the United States.
Britain's purchase of the weapons - which can be tipped with nuclear warheads - would provoke an outcry about nuclear proliferation. It could lead to a block being put on the sale by the US.
To head off protests and reassure opinion within the US, the MoD said last night: 'There is absolutely no consideration being given to the possible use of cruise missiles for tactical nuclear purposes.'
Defence chiefs are keen to deploy the missiles with conventional warheads on Britain's 12 Trafalgar class nuclear submarines to give the UK a more flexible response to a potential threat, particularly from the Middle East. Defence ministers are anxious to ensure that Britain's response to a possible threat would be credible. Ministers cancelled plans for a tactical-air-to-surface missile (TASM) fired from aircraft last year but decided to give a sub- strategic tactical role to the Trident nuclear weapon system, which Britain is buying from the US.
Britain's four Trident submarines will carry a mix of nuclear weapons, from multi- headed strategic missiles capable of penetrating hardened targets to single warheads for tactical use.
Ministers and senior defence advisers believe the Gulf war demonstrated the need for a long-range tactical missile, with a conventional warhead, capable of destroying specified targets without putting pilots' lives at risk. Buying cruise 'off the shelf' would be a simpler, cheaper option. It could be used to counter state-backed terrorism or the threat of nuclear blackmail by emerging Third World or East European states with a nuclear capability, where the use of a nuclear weapon was politically impossible.
The cruise weapons were used during the Gulf war and afterwards to back up UN demands for the destruction of Iraq's nuclear capability.
The missiles could cost hundreds of millions of pounds, but may be funded by savings from the 'Front Line First' review of support services.
David Clark, Labour's defence spokesman, said the reports of plans to cut spending highlighted the MoD's 'complete state of disarray'. He added: 'What is needed is a review of our defence and security strategy and that can only be done by having a full defence review. I urge Defence Secretary Malcolm Rifkind to drop his ideological position and conduct a review.'