Tony Blair has condemned the latest bomb attacks on the Indonesian island of Bali in the "strongest terms".
Mr Blair said he offered "full support to the people of Bali", after 26 people died in at least three blasts on Saturday, with many others injured.
British ambassador to Indonesia Charles Humfrey said one Briton was seriously injured and another "less so".
Azahari Bin Husin, a former Reading University student, is suspected of helping mastermind the strikes.
Mr Blair is writing a personal note to Indonesian President Susilo Bambang Yudhoyono offering support and sympathy.
In a statement, he said he condemned "in the strongest terms the appalling attacks in Bali."
Our thoughts are with the victims and their families.
"The UK was deeply grateful for and moved by the support and sympathy given by the Indonesian government and people after the attacks in July in London.
"We stand by Indonesia at this very difficult time.
"The British government stands ready to help in any way we can."
The British ambassador Mr Humfrey said two Britons were known to have been injured and he could not rule out with "absolute certainty" the possibility Britons were among the dead.
"We're still contacting hotels and others to make sure there's no-one missing," he told BBC News.
"There are some unidentified bodies in the mortuary so we cannot give you absolute certainty that no further Britons have been killed or injured but so far we know just of these two Britons who've been injured."
Police search for clues at the scene of the blast
The woman with dual British/Australian nationality has been airlifted off the island.
Consular staff have been to visit the two injured Britons.
The Foreign Office has set up an emergency helpline for worried friends and relatives on 0207 008 8765.
Revised travel advice on the Foreign Office website advises travellers to "exercise extreme caution at all times because there remains a high threat from terrorism".
Twenty-eight Britons were among the 202 people killed in the 12 October 2002 bomb attacks in Kuta, blamed on Islamic extremists.
Consular staff have flown to Bali from the British Embassy in Jakarta.
A senior Indonesian terror official said two Malaysian fugitives were suspected of being the masterminds behind Saturday's suicide bombings.
Major General Ansyaad Mbai named Azahari Bin Husin and Noordin Mohamed Top.
Azahari Bin Husin completed a doctorate at Reading University in the 1980s and is reportedly a bomb maker for the terror organisation Jemaah Islamiah.
He and Noordin Mohamed Top have been on Indonesia's most wanted lists since the attacks in 2002.
The blasts took place just before 2000 local time (1200 GMT).
Two blasts went off at Jimbaran - a seaside area packed with restaurants. Another was at Kuta beach, the area most popular with Western tourists.
Briton Daniel Martin, who was in a building next door to a restaurant hit in Kuta, told the BBC there was a "thunderous boom" that caused all the shop's windows to blow out. "It was just chaos."
Mr Martin said there were people lying in the streets with serious injuries, with everyone pitching in to help.
Pribadi Sutiono, from the Indonesian Embassy in London, said: "There were two [bombs] in Jimbaran beach close to the Four Seasons Hotel and one [bomb] in Kuta near the Matahari department store."
Baharat Gorasia and his wife Dipa Hirani, both 27, from north west London, had been shopping in the store during the afternoon.
Mr Gorasia said: "It's very scary. I got a chill down my spine when I heard where the bomb went off - we were in the same area just hours before.
"If we had ended up going in a few more shops we would probably have been caught up in the blast."
The Association of British Travel Agents (Abta) said it was unclear how many British tourists were in Bali.
A Thomas Cook spokeswoman said none of its 30 Britons in Bali had been caught up in the blasts but it would help any of its customers who wished to return to Britain.
Kuoni said it was trying to get information on its clients in Bali.