Key points: Blair Trident statement

Prime Minister Tony Blair has outlined his plans for the UK's nuclear weapons system. Here are the key points so far during his announcement to Parliament.

Tony Blair said the world had changed during the half century that the UK had had a nuclear deterrent, so the decision was a judgement about possible risk and threat to the country.

He said the government's judgement was that even though the Cold War was over, a new threat could emerge. He mentioned North Korea and Iran. And said there could be a possible connection between states with nuclear weapons and terrorism.

No other nuclear power was considering unilateral disarmament, and it would therefore be "unwise and dangerous" to give up Britain's independent nuclear deterrent.

The white paper explains the reasons for the decision to back the building of a new generation of submarines to carry the Trident missiles, he said. There would be a full debate with a vote in March 2007 in the House of Commons.

At the moment Britain has four submarines - one in dock, usually for 4 years, and the other three at sea or in port, but at all times there is at least one fully armed sub on patrol.

A study would take place on the feasibility of reducing the number of submarines to three.

The Vanguard subs have a service life of 25 years, with the first boat due to leave service 2017, while the second boat ends it extended life in 2024.

He said the system was fully independent and would remain so, with the UK in control of the submarines, missiles and the command chain.

The nuclear weapons budget will be 3% of the total defence budget. Investment will not be at expense of conventional defence, Mr Blair said.

The procurement and building of the submarines will be done by British manufacturers, he said.

No decision was needed now on the missiles, as it was possible to extend their life to 2042.

He would confirm, in an exchange of letters with President Bush, that the UK would "work with" the US on replacement missiles after 2042.

Retaining Britain's nuclear capability is in line with Britain's international obligations, he said, adding that he planned to reduce the UK's stock of warheads by 20% to 160 - which would be an overall halving from its peak.

Mr Blair said that while he understood the arguments of those opposed to nuclear weapons he believed that global terrorism could be sponsored by nations who want nuclear capability, and if Britain gave up its deterrent, other countries would not follow suit, but rather see it as a weakness.

Conservative leader David Cameron agreed with the timing and content of the announcement, saying the maintenance of the deterrent was important as the world continues to change rapidly and it was important to keep up Britain's guard.

The nature of the threat to Britain was unpredictable, but it could be from rogue states.

He said having a fourth submarine should not be ruled out, saying it does not have to be taken until as late as 2020.

He wanted details on the defence budget and the effect of the Trident on it.

Mr Blair thank Mr Cameron for his support and said it was important to get procurement under way now.

Lib Dem leader Sir Menzies Campbell said it was a financial, security and political decision.

He wanted to postpone a decision to 2014, cut the number of warheads to 100 and extend the life of submarines.

He questioned whether the plan was about Britain or about securing Mr Blair's legacy.

Mr Blair said it would take 17 years to replace the submarines, based on technical advice. Delaying it till 2014 could mean the replacement would not be ready.

Mr Blair said reducing the number of warheads to 160 would not damage the deterrent, but he had no basis to cut to 100.

Mr Blair said having a deterrent did not go against Britain's obligations under the Nuclear Non-Proliferation Treaty.

Keith Simpson, Conservative, asked about the US updating its weapons technology, on which the British system relies. Mr Blair said UK would have continued access to improved technology, and it was important to continue to work with the US, but the system would be independent.

Mr Blair said the submarine based system is the most suitable form of deterrent. The difficulty with an air system is that it was easier to intercept, and enormously expensive. Surface ships would also be an easy target, while land-based system, such as in the US, needs a huge area.

Michael Ancram, Conservative, questioned whether it would ever be used against rogue states, and why non-nuclear systems would not be considered.

Mr Blair said it was not possible to predict the circumstances under which nuclear deterrent would be used. The world had changed since 1940s and the threat had changed.

He was asked about advising non-nuclear states to acquire nuclear weapons. He responded that he would not recommend them to have weapons. The non-proliferation treaty had worked well, but it was recognised that Britain should be able to be a nuclear power.

He said if Britain cut its nuclear deterrent, other countries would not cut theirs nor would North Korea or Iran abandon their nuclear ambitions.

Equipment, command and control processes were totally independent and can only be fired on the command of the British prime minister, Mr Blair said.

Jeremy Corbyn, Labour, said security of the 21st century was better served by disarmament not increasing arms.

Mr Blair said the heart of the non-proliferation treaty was to allow nuclear states. Britain would not increase or upgrade, but maintain its deterrent with fewer warheads and possibly one less submarine.

He paid tribute to the skilled staff at the nuclear site Aldermaston.