"A wide range of overt and covert tactics will continue to be used in London," the department said. "Londoners should continue to go about their business as usual but we encourage the public to remain vigilant."
A police official, who spoke on customary condition of anonymity in line with policy, said the warning related to a threat from a dissident Irish Republican terrorism group.
Parts of central London close to Buckingham Palace and government buildings were closed off to traffic, with a higher than usual number of police officers on the streets.
Police said no decision had been made to increase the country's terrorism threat level as a result of the warning.
"The threat level from Irish-related terrorism has not increased and remains at substantial -- meaning that an attack is a strong possibility," the statement said.
The queen begins her groundbreaking visit to Ireland amid tight security on Tuesday. She is the first British monarch to set foot in the Republic of Ireland since 1911.