The end of the working week is the preferred time to strike as this is when many housing deals complete and solicitors move their clients’ money. The targets are often smaller, high street law firms that lack strong internet security systems.

Insurance company QBE said there have been 150 successful instances of such fraud and 10 times as many failed attempts. According to the company, the attacks started two years ago but have recently spiked; a QBE claims manager told the Financial Times that the money taken has ranged from £65,000 to 1.9m. Often only a fraction, if any, is recovered.

Previously, hackers would call law firms pretending to be bank staff investigating suspicious transactions and persuade the firm to send cash payments to their accounts, ostensibly as a test. But email is now increasingly used instead, with hackers accessing firms’ or clients’ email accounts through a fake software download. They then find a transaction that is about to complete and email the lawyers asking for the money to be sent to a different account.

The Law Society is concerned about the increase in such scams and is working with the National Crime Agency, the City of London police and the British Bankers’ Association to combat the problem.