Mourners pray for those lost in last Sunday's attack
Dependants of UK based embassy and consular staff have been told they can leave at public expense, the Foreign and Commonwealth Office (FCO) said.
The move follows a grenade attack on a Protestant church in Islamabad that left five people dead, including two US citizens.
A high state of alert was already in place after the murder of US journalist Daniel Pearl, kidnapped in Karachi in January.
Travel not advised
A spokesman for the FCO said the British move reflected "a perceived threat to British nationals in Pakistan".
He added: "Precautions have to be taken and the situation is being kept under constant review."
The FCO is also urging against all holiday and non-essential travel to Pakistan, unless those travelling are familiar with the country and have contacts such as family or friends.
"We advise against all travel to the northern areas and northern Baluchistan." a spokesman said.
"Travellers are advised to avoid inaccessible areas of the North West Frontier province from where they might find it difficult to depart promptly should the security situation deteriorate.
"The Foreign and Commonwealth Office only advises against travel when risks to personal security are explicit, or very high."