A man is being questioned after hoax e-mails were sent telling relatives of people missing following the Asian tsunami that their loved ones had died.
The government would not inform relatives via e-mail, police say
A hoaxer, claiming to be from the Thai "Foreign Office Bureau", contacted people whose appeals for help had been posted on the Sky News website.
Police said they had arrested a 40-year-old man in Lincolnshire on 31 December and seized computer equipment.
He was bailed but then rearrested on Saturday and transferred to London.
In a statement, the Metropolitan Police said the man had been detained on New Year's Eve in a joint operation with Lincolnshire police.
Scotland Yard's Specialist Crime Computer Unit is in charge of the investigation into the e-mails.
In an earlier statement, the police stressed the government would not use e-mail to inform people of a death.
The bogus address used is firstname.lastname@example.org.
Anyone receiving such an e-mail should treat it with the utmost caution, police said.
'Vital' information source
A spokesman said: "The Metropolitan Police Service would like to reassure the public that these messages are hoaxes.
"The internet is a vital source of information for many of those affected by the terrible events in South East Asia, and we would not wish to discourage people from continuing to use it."
In a statement, Sky News said: "We are disgusted at the abuse of this message board, designed for friends and relatives caught up in the tsunami disaster.
"As soon as Sky News online was alerted to the fact that a hoaxer had been emailing some of those who had posted messages, pretending to be a British Government official, it informed the Metropolitan Police and is actively co-operating with them. It also posted a story online to alert users."
The hoax e-mailer could face charges of malicious communication or causing a public nuisance.
The police have urged anyone wanting information about friends, relatives and loved ones to continue using the Foreign Office emergency number on 020 7008 0000.
The number is only for people in the UK who are concerned about the welfare of UK nationals they believe may have been in the affected areas, and not for those seeking travel advice.