The centralised registration service for e-government services was knocked out for 36 hours last week following a fire at its datacentre. The outage left more than 165,000 users unable to complete secure online transactions with government IT systems.
Failing to ensure back-up systems are in place more than six months after the launch of the service will be embarrassing to both the Government and Microsoft, which recently signed a deal to sell the system's core technology abroad.
The Cabinet Office refused to say when a contingency system will be operational but insisted it has passed through the business development stage and is "nearing completion".
"A back-up site has always been provided by the business plan but it has not been implemented yet," said a Cabinet Office spokeswoman. "We decided to prioritise the setting up of the system and then to set up the back-ups."
The outage came at one of the busiest periods of the year, as thousands logged on to file annual tax returns.