Organisers of a planned anti-nuclear march have behaved irresponsibly by not applying for road closures, police say.
The first Aldermaston march was held in 1958
They warn that the Aldermaston 2004 march, which starts on Good Friday, is in danger of putting demonstrators and motorists at risk.
Thames Valley Police say they are concerned as the march will travel the A4, one of the region's busiest roads.
Organisers say they will work with police to do all they can to ensure marchers' safety.
The march, against a new generation of nuclear weapons, leaves London on Friday after a CND rally in Trafalgar Square.
It is expected to take four days to travel to Aldermaston Atomic Weapons Establishment, passing through Southall, Slough, Maidenhead and Reading en route.
Another march from Oxford will join it as it approaches the research centre.
"We will still police it, but they are putting themselves and traffic at risk as well as our police officers," a police spokeswoman told BBC News Online.
She said organisers should have applied to the local authorities to close sections of the A4 as they passed along it, at a cost of nearly £10,000.
They raised the issue with organisers several month ago, but nothing was done.
"We're not preventing people's right to protest, but our concern is public safety," said the spokesman.
They say that last time CND walked the route, in 1988, Health and Safety legislation was not so developed.
"As such, the marches, as opposed to the protest at AWE, tread a very fine line where legality is concerned."
A CND spokesperson said: "The police have legitimate concerns about safety which we as one of the co-organisers share.
"At the same time, there's a delicate balance between concerns about road safety with the far greater risk that comes from the nuclear weapons we are protesting about."
Police have urged those coming to Aldermaston not to come on foot.