Within six to nine months, at precisely the moment when the transition from Mr Blair to Mr Brown could be under way, a Labour government will now give the go-ahead to a new generation of whatever system Britain’s defence and security chiefs deem necessary.

Yet these two men, Labour’s leader and the man who, barring accidents, will succeed him, were first elected to Parliament in 1983 and again in 1987 on Labour manifestos that promised unilateral nuclear disarmament. In 1983, when the two young MPs who were to dominate Labour’s politics arrived in Westminster, their party promised a “non-nuclear defence policy” within the lifetime of a Parliament.

In 1987 it went even further, pledging to decommission the existing Polaris system, cancel the Trident replacement, and opt for a fully conventional defence policy. Mr Blair, as an aspiring Labour politician, once flew under the banner of CND. Mr Brown was never a signed-up member and, as the MP representing the Rosyth dockyard where Polaris was refitted, was more cautious. Friends say he never went on a CND march. However, in a Commons debate in June 1984, the young MP for Dunfermline East called the Trident programme “unacceptably expensive, economically wasteful and militarily unsound”.

When he stood as Labour’s candidate in the Beaconsfield by-election in 1982, Mr Blair’s campaigners put out a leaflet saying: “Labour is the only party pledged to end the nuclear madness.”

It fell to Neil Kinnock, father of Labour’s modernisation programme, to ditch the unilateralist cause in which he was once a passionate believer. He concluded that Labour would never be electable with such a policy and in 1988 he infuriated the Left by declaring “there is now no need for something-for-nothing unilateralism”.

He sent a defence team under Gerald Kaufman to the Kremlin which happily endorsed the new approach of multilateral disarmament. In a moment of theatrical symbolism, the party’s defence team formally abandoned unilateralism at a press conference near Lenin’s tomb in Red Square.

Mr Brown’s announcement at the Mansion House last night that he will back the nuclear deterrent beyond this Parliament, which means replacement, completes the end of the long journey from Labour’s “ban-the-bomb” days. Again the Left, or what remains of it, will be angry.

Even yesterday, at Prime Minister’s Questions, Gordon Prentice, the Labour MP for Pendle, was demanding, and failing to get from Mr Blair, an assurance that there would be a parliamentary vote before the Government orders a replacement for Trident.

Mr Brown’s stance will dismay leftwingers who argue that he should spend the billions that a Trident update would cost on public services. He was already receiving dark warnings last night from MPs that he was in danger of upsetting his natural constituency in the three wings of Labour’s electoral college, the parliamentary party, the unions and the local parties. Mr Prentice said that he was dispirited by Mr Brown’s words. “He is saying all sorts of things to show there is no difference between the Blair and Brown agendas.”

But Mr Brown believes that any sign of weakness on defence and security would alienate the support he needs from Middle England.

While Mr Brown hopes that he will win the leadership without any challenge from other members of the Cabinet, he would welcome a contest with the Left if it is able to find a credible candidate. He would win overwhelmingly and define himself as reformer and moderniser.

So, Mr Brown’s announcement was part of a much wider signal that his eventual succession to the Labour crown will not mean a sudden switch away from the policies that have kept the party in power for so long. He will never allow himself to be seen as beholden to the Left.

But, equally important, it was a message to his critics on the ultra-Blairite wing of his party and to the media that Mr Blair has no monopoly when it comes to making difficult and unpopular (with his party) decisions.

GOING NUCLEAR

Times Recommends

• Third of Muslim students back killings
• Curfew tames feral yobs of Cornwall
• £3,000-a-month drug free abroad