An article published in the Guardian revealed that one of the world’s “big four” accountancy firms has been targeted by a sophisticated hack that compromised the confidential emails and plans of some of its blue-chip clients. The attack went unnoticed for months.

Deloitte discovered the hack in March this year, but it is believed the attackers may have had access to its systems since October or November 2016.

The team investigating the hack has been reviewing potentially compromised documents for six months.

Sources said if the hackers had been unable to cover their tracks, it should be possible to see where they went and what they compromised by regenerating their queries.

Deloitte confirmed it had been the victim of a hack but insisted only a small number of its clients had been “impacted”. It would not be drawn on how many of its clients had data made potentially vulnerable by the breach.