According to reports, the man allegedly posted false statements on the company's site, claiming the firm discriminated against Muslims after the London bombings of July 7.
As well as posting false statements he apparently changed the firm's share price (on the website) and cancelled company insurance contracts.
Police in London confirmed they had received allegations of blackmail and the Metropolitan Police Computer Crime Unit was liaising with Greek authorities. Since the offense was committed outside the U.K., they said, extradition would not be sought. Instead the man, described as a computer scientist, will be tried under Greek law.
Further reports suggested computers and hard disks were seized from the man's home.
In July SC reported British police were liaising with U.S. officials over the extradition of a man accused of "the world's biggest computer hack." Gary McKinnon faces an extradition hearing on October 18.