A coded bomb threat in central London has temporarily closed downmuch of the city's tourist precincts on the eve of a visit to Irelandby the Queen.

It is understood the threat was telephoned in to police by a caller claiming to represent Ireland's dissident republicans.

PallMall and other vehicle and pedestrian routes around Buckingham Palacewere cordoned off to traffic, but Scotland Yard insisted the warninggave no specific details.

Police said something suspicious wasspotted on the corner of Trafalgar Square and a controlled explosionwas carried out on an abandoned suitcase nearby.

"A bomb threatwarning has been received relating to central London today. The threatis not specific in relation to location or time," a Scotland Yardspokesman said.

"We believe the threat is in connection with dissident Irish republican terrorism."

Officers are already on high alert over the severe terrorist threat from Al Qaeda.

Itis the first coded Irish terrorism threat in more than 10 years andcomes just a week before US president Barack Obama is due to visit theBritish capital.

The Queen arrives in Ireland on Tuesday for afour-day trip, with reconciliation the theme in the first state visitby a British monarch since the republic gained independence in 1922.

Thevisit will be surrounded by a massive security operation amid threatsfrom dissident republicans to the peace established in British-ruledNorthern Ireland as a result of the 1998 Good Friday accord.

After the bomb threat in London, a spokesman for Buckingham Palace said it did not comment on security matters.

The Queen was, however, not at the palace at the time, a royal source said.

Detectiveswere also investigating a break-in at a building near the officialresidence of British foreign minister William Hague about the same timeas the bomb threat.

A spokesman would not say if the incident was related.

Specialist police officers were lowered into a sewer on a winch during the investigation, a journalist at the scene said.

Coded warnings have been traditionally used by Irish republican groups.

Therehas been a recent upsurge in dissident republican violence in NorthernIreland, with a policeman killed by a bomb in April, but Scotland Yardsaid there had been no change in the official threat level.

The last British monarch to visit Ireland was King George V, who came in 1911 when it was still part of the United Kingdom.