London, England (CNN) -- Britain raised its threat level Friday from substantial to severe, indicating a terrorist attack is "highly likely," Home Secretary Alan Johnson said.
However, he added, "there is no intelligence to suggest an attack is imminent."
He encouraged the public "to be vigilant and help police and security forces in their work."
"We know this country faces a threat from international terrorism," Johnson said. "The fact we've gone to a different level with respect to that is part of the system for protecting the public against that threat."
An international conference on Afghanistan is scheduled to begin Thursday in London. British Prime Minister Gordon Brown, Afghan President Hamid Karzai and U.N. Secretary-General Ban Ki-moon are co-hosting the meeting of foreign ministers, U.N. and NATO representatives, and members of such international organizations as the World Bank.
The increase in threat level comes two days after Brown announced Britain is expanding its security watch list and has suspended all direct flights to Yemen as part of a series of new counterterrorism measures.
Britain's highest threat level is "critical."
The U.S. Department of Homeland Security issued a statement saying Britain is "raising their measures to effectively where we are with the airport security measures that we have taken and announced over the last few weeks."
That refers to an increase in federal air marshals and enhanced screening methods in the wake of an attempted terror attack on a U.S.-bound flight from Amsterdam, Netherlands, to Detroit, Michigan, on Christmas Day.
The suspect in that attack, Umar Farouk AbdulMutallab, spent time in Yemen and is said to have acquired the device used in the foiled bombing from militants in that country. Al Qaeda in the Arabian Peninsula has claimed responsibility for the attempted attack.
The U.S. terror threat level for domestic and international flights remains at "orange," or "high."