Staff attempting to log in were erroneously told their usual passwords were "incorrect."

In an update on Monday about the incident sent to the Regulatory News Service, the company confirmed it "experienced a cyber incident primarily impacting access to internal Microsoft Office 365 applications." The nature of the incident has not been disclosed.

Capita stopped short of detailing which of its customers had been affected but Capita has some important Ministry of Defence contracts and last year took control over engineering and maintenance support of training simulators for the Royal Navy's nuclear-powered ballistic missile submarines used as part of the UK's nuclear deterrent.

Capita said in its statement, “Immediate steps were taken to successfully isolate and contain the issue," which was "limited to parts of the Capita network."

Less dangerously Capita holds contracts with the London boroughs of Barnet, Barking and Dagenham, and with South Oxfordshire council. These authorities websites displayed messages saying that phone lines for benefits, council tax and business rates call centres were down.

Capita insists that there was no evidence of any data being compromised due to Friday's cyber incident.

"The issue was limited to parts of the Capita network and there is no evidence of customer, supplier or colleague data having been compromised," the company said, adding that access to Microsoft Office 365 for the company's staff has been restored and it was working to bringing back remaining client services.