The company that handles the BBC licence fee has refused to say what data has been taken by hackers in a Russia-linked cyber attack.

Outsourcing giant Capita has admitted that hackers stole data from its servers, weeks after claiming no information had been taken.

The company, which holds around £6.5bn of public sector contracts, on Thursday said the “cyber incident” lasted for nine days before it was discovered and stopped by Capita on March 31.

Hackers were able to infiltrate Capita’s computers and steal files.

Capita said “around 4pc of Capita’s server estate” was broken into and there was “currently some evidence of limited data exfiltration from the small proportion of affected server estate.”

The company admitted that the stolen data “might include customer, supplier or colleague data,” but a spokesman refused to say who had been impacted by the breach.

The disclosure will raise concerns that taxpayer and Government information may have been taken.

Capita holds a large number of significant public sector contracts, including a £456m deal with the BBC to collect and enforce the TV licence fee.

The London-listed company also has a £2bn contract with the Department of Work and Pensions to administer disability benefits and £1.9bn-worth of contracts with the Ministry of Defence, including running training simulators for nuclear submarines.

A Russian-speaking cyber gang that calls itself Black Basta posted a cache of documents allegedly stolen from Capita on the dark web earlier this month.

Documents included scans of people’s passports and driving licences, job application letters and what appeared to be a list of bank details for suppliers who had invoiced Capita Nuclear.

Both the Cabinet Office and the National Cyber Security Centre are liaising with Capita. Cyber investigators are understood to be focused on Black Basta's hacking claims.

Capita has said it is working with “specialist advisers and forensic experts”.

The company’s admission that data had been stolen comes three weeks after the company first revealed the cyber attack.

Capita said on Friday, 31 March that it was suffering an IT outage preventing many of its 50,000 staff from being able to work because they were unable to log into its systems.

Days later the company confirmed it has been the target of a “cyber incident” but told investors there was “no evidence of customer, supplier or colleague data having been compromised”.

The initial outage impacted its Microsoft Office 365 applications, impacting O2's contact centres and call handlers for multiple local councils. 

It is understood the vast majority of Capita's customer systems are now running normally.