Army chief General Pervaiz Musharraf said Nawaz Sharif's government had been removed to prevent further destabilisation.

Mr Cook said he was "deeply worried" by the situation but added: "The military there must be under no illusion: we will strongly condemn any unconstitutional actions.

"I call on all parties to respect the constitution, the rule of law and the democratic process."

Acting British High Commisioner in Islamabad Greg Dorey: "The situation is still unclear"

The UK Foreign Office has warned Britons trapped in Pakistan to keep a low profile and has urged others not to travel to the country.

Members of Britain's large Pakistani community have been advised to postpone any planned trips until more information is available about the situation.

But British Airways said a scheduled flight to Islamabad from London via Manchester would go ahead as planned on Wednesday.

A spokeswoman said the company had been informed by Pakistani authorities that Islamabad Airport was being fully reopened for international services.

Robin Cook said he was "worried" by the situation

She said: "We have got crew and there are over 300 passengers who want to fly. We are sure it is safe to fly there, it seems to be calm."

The coup by Pakistani soldiers began after Mr Sharif decided to sack Gen Musharraf.

A Foreign Office spokesman said British officials in Islamabad and Karachi were monitoring the situation and would be working to provide "consular assistance" to British nationals in the country.

There is no early indication of how many Britons might be in Pakistan, but the spokesman said many people there had dual British-Pakistani nationality.

The BBC's Mike Williams reports from Southall in London, where some people are expressing concern, while others are relieved

"British nationals and their families in Pakistan should keep in touch with consular wardens and the British High Commission," the spokesman said.

There was no immediate explanation for the army chief's sacking, but in recent weeks there have been reports of a rift between the army and the civilian government.

'Coup no surprise'

Tariq Azim, Secretary General of the UK Pakistan Cultural Foundation, told BBC News Online that the coup would not be cause for alarm among those with relatives in Pakistan.

He said: "The news will not come as much of a surprise to Pakistanis living here.

Prime Minister Nawaz Sharif is under house arrest

"The army has been a back seat driver in Pakistan for many years - it has always been involved in political affairs.

"But it is a very disciplined army. I don't believe the situation is in danger of getting out of control."

Britain's acting High Commissioner in Islamabad, Greg Dorey, said he had been in contact with "a range of people" in Pakistan over the coup.

He said: "We are making clear that we are concerned about the situation, also that the military have got to understand that we would strongly condemn any unconstitutional actions, so we are urging everybody to return to the democratic process as soon as possible."

'Calm' in Islamabad

Peter Elwood, director of the British Council in Pakistan, has told officials in London that staff at their five centres were safe.

The council has eight staff, 12 teachers and 19 consultants from the UK working from offices in Islamabad, Gilgit, Karachi, Lahore and Pashwar alongside 180 local staff.

The Foreign Office has set up a telephone hotline for people planning to travel to Pakistan or for those inquiring about British nationals there.