The Metropolitan Police has appealed for those whose loved ones turned up alive following the Indian Ocean tsunami to inform authorities.
The Met Police is co-ordinating details about missing Britons
A Met Police spokesman urged those who had reported a person missing to get in touch to keep records "up-to-date".
The Met is co-ordinating details about missing Britons and has sent 40 forensic experts to Thailand and Sri Lanka to try to identify the victims.
Forty two Britons are confirmed dead and dozens are missing after the quake.
The total number of confirmed deaths following the tsunami stands at about 140,000.
Since the disaster on Boxing Day, concerned callers have rung the emergency telephone number to report that they have not heard from a friend or relative they knew was visiting the stricken area.
Met Police Deputy Assistant Commissioner John Yates said: "In the relief of knowing that a friend or relative is safe, people who originally contacted us may not have remembered to let us know.
"In order that Foreign & Commonwealth Office records are up-to-date, we would urge the people who originally called us to ring again".
A spokesman added: "The only way that we can effectively track the number of missing people is if people call us back.
"This will ensure that our resources are deployed as effectively as possible."
The emergency telephone is 020 7008 0000, it should not be used for seeking travel advice
Travel advice can be found on the Foreign Office website at www.fco.gov.uk or by calling 0870 6060290
Flight information should be obtained from travel and airport operators
Anyone who called about a missing relative or friend that has since turned up alive is asked to report the person's return by calling 020 7008 0000.
On Wednesday the Queen visited an emergency call centre in London where workers are helping people searching for loved ones missing since the Asian tsunami.
She and Prince Philip spoke to staff who have been working up to 16-hour shifts at the casualty bureau appeal centre in Hendon since Boxing Day.
The call centre suffered difficulties in the immediate aftermath of the quake as it attempted to cope with the high levels of calls - sometimes up to 600 an hour.