A bomb threat from Irish militants was investigated by Scotland Yard anti-terrorism officers, a day prior to Queen Elizabeth visit to Ireland.
A bomb threat related to Central London has been received on Monday. The threat is not specific in relation to location or time, London police said.
“The caller had used a codeword known to the police, lending credibility to the threat,” a security source said.
A bomb threat has temporarily closed down much of the city's tourist area, Mall, a broad avenue leading to Buckingham Palace, Queen Elizabeth's residence, Police said.
It is the first Irish terrorism threat in 10 years just a week before US President Barack Obama visit to British capital. In March 2001 the last Irish-related attack was reported when a car bomb exploded outside BBC's London headquarters.
Despite a 1998 peace deal mostly ending Northern Ireland's three decades of conflict, violence by dissident Republicans opposed to the peace process has been increasing in the British-ruled province.
We believe the threat is in connection with dissident Irish republican terrorism, said Police. Officers are on high alert over the severe terrorist threat from al Qaeda.