The United Nations says alleged British bugging of Secretary-General Mr Kofi Annan's office, if true, violates international treaties and should be stopped immediately.
"We are throwing down a red flag and saying, if this is true, then stop it," UN spokesman Mr Fred Eckhard told reporters. "It is not good for the United Nations' work and it is illegal."
He said that "the secretariat routinely takes technical measures to guard against such invasions of privacy, and those efforts will now be intensified." He cited treaties on the inviolability of UN premises.
Mr Eckhard was responding to comments by Ms Clare Short, former international development secretary, who said that British intelligence agents spied on Mr Annan ahead of the US-led invasion of Iraq last March.
Ms Short told Channel 4 news that the United States had been blamed for eavesdropping on offices of some Security Council members last year so "I think it's right that people in Britain should know that the UK is doing that to the secretary-general."
The United Nations usually does not respond to such charges but the world body was clearly startled by Ms Short's claims because she was a high-ranking official at the time the alleged bugging took place. Short resigned from the government after the war.