There's a man in Downing Street, close to Tony Blair, who is rather keen on NMD, the national missile defence system which George W Bush has said he will deploy. He believes there will soon be so many loose missiles in rogue states that appropriate counter-missiles may be essential. His voice won't be decisive.

Most official British opinion is strongly, if discreetly, opposed to NMD. But a straw flutters in the wind. An accommodation is possibly being prepared. What Bush proposes and Colin Powell disposes, Tony Blair may decide he cannot oppose. Not far over the horizon, the ground is opening up for the most awkward struggle in the modern history of Anglo-American relations.

General Powell upped the ante on Monday, his first outing as Bush's secretary-of-state designate. The ambiguities of the Clinton attitude to NMD are already being dispelled. "We're going to go forward," Powell firmly said. There would be tough negotiations with those countries that "don't yet understand our thinking with respect to national missile defence". The thinking is very committed. Bush made that clear several times in the election campaign. Powell, though once listed as a Pentagon man who feared NMD would take money away from more treasured military ventures, is now the voice of an opposite decision already made in principle.

His statement coincided with another. A CIA study-project, making an assessment of the next 15 years, asserts that the threat to the mainland US from nuclear missiles, among other weapons, is greater than it was for most of the cold war. This will, the CIA thinks, get worse. Terrorists and rogue states will have the power and motivation to launch attacks that produce "mass casualties". This wasn't said to justify NMD. But it nourishes the mind-set that is demanding a new $60bn defence programme, irrespective of what might happen in the rogue states in question.

Both North Korea and Iran arguably show reason for mild optimism. Iraq could soon be the solitary credible danger. No matter. No matter, either, that Russia, with whom the US has an anti-ballistic missile treaty, is fiercely opposed. Russia no longer matters, argues Richard Perle, a close Bush associate. The lead-time to build NMD is long, the world is usually changing for the worse, and the Washington voices prepared to speak up against unilateral defence may soon be hard to hear.

At present, Downing Street professes to be quite comfortable. Blair got Clinton and Russian president Putin talking about NMD. He wants to be, as ever, the contact man, mediator, bridge. Since the election, his people have talked to Powell and, at greater length, to Condoleezza Rice, Bush's new national security adviser. The talks have apparently been more reassuring than Powell's public statement. Rice vowed to proceed with caution, sensitive to allied worries.

Maybe she will. These advisers matter hugely. Bush's personal contribution to all complicated subjects will be far less creative than Clinton's. He will be heavily adviser-dependent. Rice says she is interested in sea-based defence missiles aimed to hit rogues at the point of launch, rather than the scheme Clinton half-backed which depended on the hit being made in space. The work required to switch schemes would be massive. But a boost-phase system would at least not depend on British and Danish tracking stations.

As presently conceived, NMD proposes a new hi-tech skyscraper on Fylingdales moor, a project which, as British ministers have tried to explain in Washington, would be relentlessly picketed not only by CND and Greenpeace but by the north Yorkshire branch of the Women's Institute, furious at defilement of the countryside.

Under Clinton, British and European objections to NMD fed into an administration inclined to take them seriously. They weren't limited to the region's interest in not being targeted, as helpless accomplices to American unilateralism, but put most weight on the global instability provoked by a new arms race.

If NMD ever did happen, Russia and especially China would feel obliged to counter it. Whitehall, like Paris and Berlin, has made these arguments copiously. A quiet turmoil of alarm has gripped the Foreign Office and Ministry of Defence for months.

They now face a Washington being peopled by voices that make a different analysis: scornful of Russia, arrogant about China, intolerant of European sensitivities, overwhelmingly impressed by the case for defending US territory - and confident that the practical failure of NMD so far, like the untested capacities of a boost-phase project, are mere blips on the relentless American path towards technological mastery.

One answer being given to the choice between space- attack and launch-attack systems is that both should be pursued. Though it remains to be seen how Congress will react to the financial burden, missile defence is not guaranteed to be one of the aspects of the Bush programme that founders on a bitterly divided legislature. My conversations tell me that the commitment to build and deploy is likely to be made: much more likely, at any rate, than not.

This will present Tony Blair with a challenge he has not so far faced. His time in office has been spent with a fellow spirit in the White House, someone to whom he grew close. We may be sure he will attempt to attain real intimacy with Bush, and he's likely to be more successful at it than John Major was with Clinton.

Mr Blair makes a good fist of being friends with every species on the political spectrum, the only exception being a certain kind of Tory politician. He will strain every muscle to get on with Bush. And Bush, never having visited a major European country, will be more than usually susceptible to the charms of a land whose language he understands, whose history his father knows, and which matters more than Aznar's Spain, the only other contender.

Blair, in other words, could become a safe haven for the Texas ingenu. His personal instincts will be to do whatever he can to help Bush; and now he has a voice or two whispering words of encouragement to persuade him that NMD may not, after all, be as dangerous as the defence and diplomatic establishment has been telling him. There are, after all, a lot of loose missiles about. Maybe NMD, if the Americans can produce one that works, is just what we need. Maybe the big picture says that it's the prime minister's duty to rise above conventional wisdom.

If this became Blair's formal position, it would create his deepest rupture with continental Europe. A moment of truth almost certainly beckons.

Will NMD become a pretext that requires one more affirmation of the old special relationship? Or the project that at last obliges Britain to recognise she cannot continue as the compliant poodle? Without doubt it will be the issue that shatters the Blair axiom that there's no choice to be made.

hugoyoung@compuserve.com