"A bomb threat ... has been received relating to central London today. The threat is not specific in relation to location or time," London police said in a statement.
A security source said the caller had used a codeword known to the police, lending credibility to the threat.
The warning came on the eve of the first visit to Ireland by a British monarch in a century and a week before U.S. President Barack Obama makes a state visit to London.
Police said earlier that a security alert had led to the closure of the Mall, a broad avenue leading to Buckingham Palace, Queen Elizabeth's residence, but refused to say what had prompted it.
A Northern Irish republican militant group, the Real IRA, told Queen Elizabeth, head of state of Ireland's former colonial master, last month she was not welcome on Irish soil.
British member of parliament Patrick Mercer said last year he believed militants from Northern Ireland hoped to stage attacks on the British "mainland".
The last Irish-related attack in London came in March 2001 when a powerful car bomb exploded outside the BBC's London headquarters. Police say the Real IRA, a republican splinter group opposed to the IRA's ceasefire, was behind the blast. One man was wounded.
The IRA mounted several bombing operations in England during its campaign. Such attacks were more difficult than actions in northern Ireland, since the guerrillas lacked support networks; but they arguably had more impact on the British government.
The IRA staged its most spectacular attack in 1984 when it blew up the Brighton hotel where Prime Minister Margaret Thatcher was staying, along with her chief ministers. She narrowly escaped, but five people were killed.
Britain's interior ministry said in a statement: "We face a real and serious threat from terrorism ... There is a continuing need for vigilance and the public should report any suspicious activity to the police."
The police said the threat of Irish-related attacks was considered lower than the overall threat to Britain from international terrorism which remains at severe, the second highest level on a five-step scale.