NOW it’s official: Britain will replace Trident. There was little doubt that at some point Tony Blair or Gordon Brown would break this news to the Labour Party. The doubt was which one and when.

But the complexity of the threats which Britain now appears to face, and the controversial relationship with the US, have given their opponents a few more arguments.

Yet they are unlikely to prove powerful enough to kill the programme, given the importance that Britain’s nuclear capability has to its international standing — and signs of broad public support.

If Britain wants to remain a nuclear power, it needs to pick a replacement for Trident in this Parliament; the 2003 defence White Paper made that clear. The claims by assorted ministers that they had not started to think about a replacement have been absurd.

They may still, quite reasonably, have not decided on its form: whether it still should be based on submarines, or whether an air-based or ground-launched system might be better. But they will have to decide that briskly by 2008, to allow time for development before Trident retires in 2024.

The anti-Trident case breaks into three parts:

All kinds of figures for the cost of a replacement whirl around; generally, if wielded by the anti-Trident camp, they are upwards of £20 billion. These don’t mean much until the nature of a replacement is clear and, so far, it is not.

On more solid ground, maintaining Trident as it is now costs between 3 per cent and 4 per cent of the military budget, or about £1 billion a year.

This argument has most purchase with those who think the money would be better spent on development or health or, really, anything else. But while the anti-Trident camp regards this as powerful ammunition for its cause, the row is so emotive that opinions are never likely to turn on mere finance.

Many argue that because Britain relies on the US for technical help in maintaining and operating Trident, Britain is not an independent nuclear power.

What is more, it inextricably entangles British military strategy with that of the US, they argue.

Never mind that Ernest Bevin, as Labour Foreign Secretary in 1946, declared: “We’ve got to have this thing over here whatever it costs. We’ve got to have the bloody Union Jack on top of it.” It looks more US than British now, critics argue.

This complaint has inevitably become entangled with the opposition to the Iraq war and the Bush Administration. But it looks odd and parochial.

To say that Britain makes use of US technical help — or even that any use of nuclear weapons without US support would be inconceivable — is a long way from saying that it might as well not have them.

Britain’s weight as a nuclear power is obvious — in the wrangling with Iran over its nuclear ambitions, in US relations and, more subtly, within the EU.

It is true that Islamic fundamentalists bent on attacking Britain will not be “deterred” by its possession of nuclear weapons, the classic case for having them.

But that is not the only threat Britain faces, as the nuclear ambitions of Iran and North Korea have shown.

In formally starting the battle, Brown has reason for hope: loathing of Trident has run far deeper in the Labour Party than in the public at large, where a desire for Britain to keep its influence in the world still seems high.

Times Recommends

• UK regional accents thrive against the odds
• ‘Scarecrow’ wind farms put rare birds to flight
• What an anti-climax: G-spot is a myth