Foreign Secretary Jack Straw's announcement followed news that the British High Commission in the Pakistani capital of Islamabad was closing all its visa offices because of threats.
Mr Straw urged UK nationals to consider leaving Pakistan - and said that for the time-being there would be no routine visa service for people wanting to visit the UK.
"We will begin reducing the number of staff and dependents in Pakistan with immediate effect," Mr Straw said.
"We are now advising against all but essential travel to Pakistan and then only where there is a compelling reason and where security is assured.
"We are also advising British nationals in Pakistan to consider leaving."
The numbers of diplomats, staff and dependants attached to the British High Commission in the capital Islamabad will be cut from around 210 to about 80.
The Deputy High Commission in Karachi will see a cut from 36 to 10, while the office in Lahore, which currently has about 10 Britons attached to it, will close.
Target concerns
Mr Straw did not say where the threats came from, but pointed to a recent series of "terrorist outrages".
The three visa offices in Pakistan's main cities have been closed, and the British Council has shut its doors to the public.
Any remaining British citizens are being warned to take particular care over their own personal security.
There had been fears the British community in Pakistan could become a target for Islamic extremists, particularly once the British forces joined the American-led operation inside Afghanistan against al-Qaeda and Taleban.
There have been three attacks on foreigners since the beginning of the year.
The American journalist, Daniel Pearl, was kidnapped and subsequently murdered.
Then an Islamabad church used by the international community was attacked, killing among others an American diplomat's wife and daughter.
Two weeks ago, a car bomb exploded next to a navy bus in Karachi, killing 14 people, 11 of them French technicians.