David Warren, the UK’s ambassador to Japan, has conceded that “given the scale of the disaster” his staff are preparing for bad news.
More than 17,000 Britons live in Japan, including hundreds along the north east coast where whole communities have been wiped out.
Around 450 Britons have failed to make contact with their families, though many of those are likely to be alive and unable to get messages out because of damage to telephone networks.
The Foreign Office has “serious concerns” for up to 50 Britons known to have been living in the worst-hit areas.
Mr Warren has travelled to Sendai, at the heart of the tsunami zone, to set up an operations base of consular staff who are trying to find British survivors and notify their families.
He told the BBC: “Given the scale of the disaster here, while we have no confirmed British casualties as yet, we must be prepared to hear news of them.
“It is still very early days in terms of clarifying the total numbers of people who have died.
“We have a list of British nationals who are unaccounted for, which we are working through to establish their safety.
“We are continually being contacted by people who were in the earthquake zone but are safe and well.”
For further inquiries, the Foreign and Commonwealth office can be contacted on 020 7008 000 or email@example.com