President Bush has agreed to discuss a further extension of the lifespan of the UK's Trident nuclear missiles.
Trident is a submarine-based nuclear weapons system
The US already plans to extend the life of the missiles, which both the US and the UK use, into the 2040s.
But in an exchange of letters Mr Bush has agreed to Tony Blair's request to discuss "a further life extension for UK purposes" of the Trident missiles.
This would mean that they could last to the 2050s, matching the lifetime of the UK's proposed £20bn submarine fleet.
When he launched the white paper on The Future of the UK's Nuclear Deterrent earlier this month, Mr Blair told MPs the D5 Trident missiles could have their lives extended by the US to 2042.
The UK, he said, would then seek to collaborate with the US to ensure that its next generation of missiles would be usable in the UK's proposed fleet of nuclear submarines which are expected to be in service from 2024 to the 2050s.
But in a letter to the White House to confirm these agreements, Mr Blair also says there would be merit in the UK having the opportunity "to discuss a further life extension - for UK purposes - of the D5 missile, to match the potential out of service date of our new submarines".
TRIDENT MISSILE SYSTEM
Missile length: 44ft (13m)
Weight: 130,000lb (58,500kg)
Diameter: 74 inches (1.9m)
Range: More than 4,600 miles (7,400km)
Power plant: Three stage solid propellant rocket
Cost: £16.8m ($29.1m) per missile
Source: Federation of American Scientists
In his reply President Bush invites the UK to "participate at an early stage in any program to replace the D5 missiles or to discuss a further life extension - for your purposes - of the D5 missile to match the potential out-of-service date of your new submarines".
President Bush confirms that "any successor to the D5 system should be compatible, or capable of being made compatible with, the launch system for the D5 missile, which you will be installing into your new submarines".
The white paper outlined plans to spend up to £20bn on a new generation of submarines for Trident missiles.
The current Vanguard nuclear submarines need to be replaced by 2024 if the UK is to continue with a submarine based Trident nuclear system, and Mr Blair says in his letter to Mr Bush that "we have decided that we will replace" them.
The white paper set out the decision Mr Blair had taken on Trident, with a three month consultation period now taking place before MPs vote on the plan in February or March.
But Liberal Democrat defence spokesman Willie Rennie said the letters showed Mr Blair had already made the decision.
"This is typical of his attitude to Parliament," he said.
"One legacy the prime minister does not want to leave behind is a decision on this country's future nuclear deterrent for the next 30 years that was made without a full and proper parliamentary debate and vote."
A number of Labour MPs oppose the government's plan, but the Tories say it would be "crazy" not to have UK nuclear weapons. The Lib Dems want a decision on the submarines put back to 2014.
Critics believe the money would be better spent elsewhere and that Trident belongs in the Cold War era, not at a time of threats to the UK such as international terrorism.