The Queen sent a message to Pakistan's president expressing her "heartfelt sympathy" at the devastation, a sentiment echoed by Prime Minister Tony Blair.
In her message, the Queen said: "I was shocked to hear of the devastating earthquake which has caused such destruction and loss of life in Pakistan.
"I send my heartfelt sympathy to you and the people of Pakistan, especially to the bereaved and injured and all who have been affected by this terrible disaster."
Mr Blair promised the UK stood ready to provide more assistance " as and when required".
He said: "We are immensely proud of our close ties to Pakistan, made even closer by the large population of British citizens, who trace their origins to the Kashmir region.
"Such ties make the growing number of casualties even harder to bear.
"Our thoughts go out to all Pakistanis."
The Government has contributed more than £500,000 in direct assistance, and their Irish counterparts have pledged one million euro (£615,000) and the EU £2.5 million.
The Disasters Emergency Committee, an umbrella organisation of some 13 agencies which responds to major disasters and emergencies, is planning to meet today to discuss their response to the disaster.
Britain has already sent teams of aid and emergency workers, some of whom have been deployed at a collapsed tower block in Islamabad, while others are en route for Kashmir.
The Pakistan High Commission is dealing with relief assistance, and providing emergency visa services in their London offices and at Heathrow, Birmingham and Manchester airports to help those in a rush without the required paperwork.
* Those worried about their relatives can call the Foreign Office information line on 020 7008 1500.