Police are planning counter measures against suicide bombers when Prime Minister Tony Blair and his Cabinet come to Brighton and Hove.

All police leave has been cancelled as Sussex braces itself for what could be the biggest security operation yet mounted.

The Labour Party conference is six months away - at the time of the 20th anniversary of the IRA bombing of The Grand hotel - but security preparations have already begun.

Sussex Police and military experts started detailed planning late last month.

An air-exclusion zone, monitored by the RAF, will be thrown up over the city and the Royal Navy will be on alert. However, the main focus of attention will be on the the threat posed by suicide bombers.

With Mr Blair and his Cabinet virtually under one roof, Sussex Police and intelligence services are treating the conference as a prime target.

Home Secretary David Blunkett has said a terrorist attack in Britain is "inevitable" and the police are taking no chances.

Mr Blunkett's office is pumping an estimated £2 million into the operation.

While visible security in the city will be kept to a minimum - the seafront will stay open for vehicles and pedestrians - there will be much tighter controls on visitors.

Anyone trying to get passes to the Island Site, the area round the Grand Hotel and Brighton Centre where the conference will be held, will be looked at in more depth than ever before. Intelligence agents are keeping tabs on suspects' movements and operations will be stepped up during the five-day conference, which starts on September 26.

Precise details of Operation Otter are top secret but every available police officer in the county will be working to protect the Island Site while maintaining policing levels elsewhere.

Brighton and Hove City Council leader Ken Bodfish said: "In the aftermath of the Iraq war we have to have a high level of security and the council is supporting this operation."

He added that party conferences were very lucrative for the city and he was sure disruption to traffic and pedestrian movement would not be significant.

Chief Superintendent Phil Clarke, in charge of Otter, said vetting of delegates, visitors and members of the Press would be the most comprehensive to date.

He said: "The degree of checking will go deeper and with some people very deep indeed. If there is any doubt, they will be refused passes."

An aerial threat is being studied but it is considered unlikely.

Terrorists more often pick high-profile buildings.

Mr Clarke said the level of security protecting the House of Commons and other Government buildings would be applied to the Island Site.

Suicide bombers were the key threat but were not the only concern.

There are still active IRA splinter groups who would relish the chance of repeating the 1984 bombing of the Grand.

Military explosives experts and police will comb every room in every building and street manholes will be sealed.

Sussex Police have a "mutual aid" agreement which could enlist officers from neighbouring forces if the need arises.

It is not known whether any demonstrations are planned but if Brighton and Hove Albion's stadium plans for Falmer are rejected, supporters could make their anger heard.

The seafront Kings Road and nearby West Street and Russell Road will be kept open for traffic but Cannon Place will be restricted to one-way traffic.

Mr Clarke said with so many police on the streets the level of ordinary crime normally took a nose dive.