The Post Office has appointed an external investigator to examine its Horizon accounting system.

Forensic accountancy firm 2nd Sight has been called in to conduct an independent review of 10 outstanding cases involving subpostmasters, after issues with the system were raised by MPs and law firm Shoosmiths.

Before beginning the full investigation into the cases, 2nd Sight will meet with James Arbuthnot MP to reassure him of their independence on the issue.

The Horizon system has attracted controversy since it was introduced to the network in 2002. Since than more than 100 subpostmasters have blamed the system for glitches which have resulted in missing money - up to £35,000 in some cases - that retailers have had to pay back or face criminal charges.

The Post Office is adamant that the system isn’t at fault. “Over the past 10 years, many millions of branch reconciliations have been carried out with transactions and balances accurately recorded by more than 25,000 different subpostmasters,” said a Post Office spokesman. “The Post Office has no hesitation in agreeing to an external review of these few individual cases that have been raised with us by a number of MPs.”

General secretary of the National Federation of SubPostmasters George Thomson also gave the system his backing. “We continue to have complete confidence in the Horizon system, which carries out hundreds of millions of transactions every week at 11,500 Post Office outlets across the country,” he said.