Irish republican dissidents have issued a bomb threat for central London, Scotland Yard has confirmed.
The Mall and other central London streets were closed on Monday morning although a link has not been confirmed.
The threat level for Northern Ireland-related terrorism has not been changed.
The Met Police statement said: “All officers have been advised to be highly vigilant to ensure the safety of London.”
A Home Office spokesman said the UK faced a “real and serious threat from terrorism”.
“The overall threat level from international terrorism remains at severe which means that an attack is highly likely.
“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.”
The Metropolitan Police have confirmed they are investigating a break-in at Carlton Gardens but could not confirm whether or not the incident was linked to the closure.
The building is adjacent to the foreign secretary’s residence, where William Hague is believed to have been at the time.
Roads around Northumberland Avenue were also closed while a bag was destroyed in a controlled explosion. The roads have now reopened.
Police said the bag was no longer deemed suspicious.