The target co-ordinates have been removed from the missiles carried in Polaris submarines, and the tapes immediately available now would direct the missiles to points in the deep ocean.
The move fulfilled an agreement made between John Major and Boris Yeltsin in Moscow on 15 February. The United States and Russia made a similar deal in January and American missiles were also 'detargeted' on Monday.
It coincided with moves to obtain a ruling in the World Court that the use of nuclear weapons is illegal. Submissions are due by 10 June. The nuclear powers have exerted strong pressure to prevent such a ruling, as it would undermine the old-fashioned 'deterrence theory'.
Ukraine, which inherited nuclear weapons from the former Soviet Union and is still the world's third-largest nuclear power, broadly supports the move to make their use a 'war crime', but retains its stocks partly as a bargaining chip against Russia.
According to the Ministry of Defence, 'the guidance computers on UK strategic missiles no longer routinely hold targeting information'.
Defence sources said it was ludicrous to suppose the missiles were now pointed somewhere else - Iraq, for example.
However, if an international crisis was to develop, tapes with suitable targets could be made available very quickly: if not carried on the submarine, they could be delivered quickly. Since other potential nuclear targets lack the Russians' ability to track ballistic missile firing submarines on the surface, this would not compromise the invulnerability of the deterrent.Reuse content