A bomb threat for London has been issued by Irish dissidents in the first coded warning outside Northern Ireland in 10 years, officials say.
Whitehall officials said the call with the coded warning came from a number in the Irish Republic on Sunday evening.
The threat level for Northern Ireland-related terrorism on the British mainland remains at substantial.
Streets around The Mall were closed on Monday morning in a security alert which police said was unrelated.
The warning comes ahead of the Queen's historic visit to the Republic of Ireland on Tuesday, amid a massive security operation.
The Metropolitan Police said the threat was not specific regarding location or time.
A Scotland Yard spokesman said in a statement: "All officers have been advised to be highly vigilant to ensure the safety of London."
The public were urged to go about their normal business but to look out for "unusual activity or behaviour".
Earlier on Monday, The Mall and Carlton House Terrace were closed after a security alert was sparked by a break-in at Carlton Gardens.
The break-in is thought to have taken place at 2 Carlton Gardens, which houses the Institute for Government think tank.
That is adjacent to the foreign secretary's residence, where William Hague is believed to have been at the time.
'Within the know'
Pictures from the scene showed a van of search dogs, and the Met's underwater and confined spaces search team.
Roads around Northumberland Avenue were also closed while a bag was destroyed in a controlled explosion. The roads later re-opened.
BBC security correspondent Frank Gardner said that the call with the coded bomb threat "was not directly related to the break-in near the foreign secretary's house in Carlton Gardens or the abandoned suitcase near Trafalgar Square, but both were taken very seriously in the light of the bomb threat".
BBC security correspondent Gordon Corera said the level of security in the Republic of Ireland would make it difficult for dissidents to disrupt the Queen's trip there.
"There was always a concern that timed with the visit there was the possibility of something happening in Northern Ireland, or perhaps in Britain," he said.
'Real, serious threat'
The former security minister, Lord West, said that if a coded message was being used, then it would have come from someone "within the know, within the circle".
He added: "We know very well that this very, very tiny number of dissidents who can cause mayhem way beyond the scale of the numbers there, are absolutely set on trying to revert to the bloodshed, mayhem and bloody massacres of the past."
A Home Office spokesman said the UK faced a "real and serious threat from terrorism".
"The threat level to Great Britain from Northern Irish-related terrorism remains at substantial, which means that an attack is a strong possibility.
"There is a continuing need for vigilance and the public should report any suspicious activity to the police."