A British man is now thought to have died in the bomb blasts on the Indonesian island of Bali on Saturday.
Terror group Jemaah Islamiah has been blamed for the attacks
The Foreign Office said it was "highly probable" a UK national died in the attacks which killed 19 people.
But it said it could not confirm the death before a coroner's report and formal identification on Thursday.
However, Australian company Tunnel Mining issued a statement "fearing the worst" for British-born founder Colin Zwolinski, who was holidaying in Bali.
Previously two Britons were believed injured in the blast but officials thought no British holidaymakers had died.
High terror threat
A Foreign Office spokeswoman in London said: "We are still waiting for the coroner's report before we can formally identify anybody."
Mr Zwolinksi, the managing director of Tunnel Mining, was holidaying in the region with his wife Fiona. Her fate was also unconfirmed but the company said it was "expecting the worst".
Many holidaymakers were among those injured in the bombings
Both were thought to be in their 40s and Mr Zwolinski was said to have had dual British and Australian nationality, the Press Association reported.
The company statement said the couple's children were safe and back home in Newcastle, New South Wales, Australia.
The firm's website described Tunnel Mining as "a successful, 100% Australian-owned company based at Gateshead, New South Wales, with branches in Indonesia, India and Thailand".
It said it had been established by Mr Zwolinksi to provide support to the mining and construction industries by supplying equipment and professional expertise.
Most of those killed in the blasts were believed to be Indonesian, although people from Australia, Japan, South Korea and the US were also among the confirmed victims.
The Foreign Office said at the weekend it believed none of the estimated 1,000-plus British holidaymakers on the island were among the dead.
But it sent a rapid response team to Bali to search hospitals and the surrounding area to make sure no more Britons had been caught up in the blasts.
The team has also been working to ensure travellers are safe, brief them on the situation, provide reassurance and help with any early departures.
The number of British tourists visiting Bali is down on recent years following the bomb blasts of October 2002 in which 202 people were killed by suspected Islamic extremists.
The Foreign Office urges travellers to Bali to "exercise extreme caution at all times" because of the high threat from terrorism.
Indonesian anti-terror officials said the remains of three bombers were found at the blast scenes.
Authorities on the island say the attacks appear to have been carried out by regional militant group Jemaah Islamiah - which was also blamed for the 2002 attacks.
A helpline has been set up for worried relatives on 0207 008 8765.