London, Dec. 4 (Reuters): Prime Minister Tony Blair pledged Britain would keep its nuclear arsenal well into the 21st century today, saying it would be unwise and dangerous for it to give up its deterrent unilaterally.

Unveiling a policy document in parliament, Blair said the government planned to order up to four new, nuclear-armed submarines to replace its existing nuclear defence.

In a concession to dozens of legislators in his Labour Party who oppose a new nuclear weapons system, Blair said Britain would cut its nuclear warheads by 20 per cent to fewer than 160 and might reduce its submarine fleet to three from four. The decision keeps Britain in a nuclear club comprising all five permanent members of the UN Security Council.