Anxious British relatives say they have been unable to hear news of loved ones who may have been caught up in the Asian earthquake and tsunamis.
British tourists have gathered at Phuket City Hall to wait for news
Some said they could not access the Foreign Office emergency helpline, as reports emerged of the devastation caused in the region.
A Tory MP said he would ask Jack Straw to explain to the Commons why relatives have not been kept better informed.
The Foreign Office said staff were working all night to field calls.
At least 12,000 have died in seven countries as a result of the underwater earthquake, which sent giant waves crashing onto coastlines across the region.
The 9.0 magnitude quake - the worst in the world for 40 years - struck under the Indian Ocean off western Indonesia on Sunday morning.
Two Britons have been confirmed dead, one in Thailand and one in the Maldives.
The Foreign and Commonwealth Office set up a crisis line on Sunday morning for relatives and loved ones to call for information on 020 7008 0000.
An emergency helpline bureau was set up at the Metropolitan Police, taking hundreds of calls in just a few hours.
Commander Ronald McPherson, in charge of the operation, said the number of calls they had received was among the highest they had experienced.
Tsunamis brought devastation to several countries in the region
"Since we opened the lines have been permanently at capacity... We will stay open until we are satisfied we have done everything we can.
"We know there are a number of casualties. What we don't know is how many are UK nationals."
But many friends and relatives are getting increasingly angry as they continue to have trouble getting through to the helplines.
Rob White, from Devon, who has friends in Sri Lanka, told the BBC News website: "After a million calls I finally got through to the FO number, [only] to be put on hold for 30 minutes and then cut off. What kind of communication is that?"
William Owen from Colwyn Bay, whose daughter is also in Sri Lanka, said: "Nobody can give me any answers. There are many injured people and some have lost relatives but not one person has heeded my plea for information."
Kevin Tunbridge, of Suffolk, told the Press Association he was looking for news of his son and his son's girlfriend in Thailand: "I have tried to contact the Foreign Office and the Bangkok Embassy all day, but all I have reached is a recorded BT message. All we want is some information."
Gerald Howarth, Conservative MP for Aldershot, has tabled a Commons question to the foreign secretary asking "what went wrong" that left what he said were "inadequate" arrangements in place at the Foreign Office.
"Even MPs could not get through to get information on behalf of their constituents. There must thousands of people in the country in this plight.
"I think the Foreign Office should have been far better prepared for this sort of thing. The system in place is plainly inadequate.
The hunt for survivors continues as the clean-up begins
"People have been trying to get through to this number for hours on end without success."
Mr McPherson urged people to be patient, adding: "We understand how concerned they will be about their relatives."
A Foreign Office spokesman said staff in London and Thailand were working through the night to answer inquiries.