The Gateway Review process is meant to be mandatory, particularly for high-risk, 'big bang' projects such as tax credits that have a single go-live date for the whole system.

But sources close to the project told Computing the Revenue skipped the first three stages - called Gateway Zero, One and Two - which cover essentials such as business justification and procurement strategy.

The six-stage process, overseen by the Office of Government Commerce, is designed to monitor projects at key stages to guard against failure.

Paymaster General Dawn Primarolo told the BBC's Inside Money programme last month that the Revenue had not been warned about problems with the IT, and that the project had the OGC's full sanction.

'The report the OGC did on our readiness going into April was that it was an exemplary example,' she said.

But the OGC was not involved with tax credits until the end of last year - at Gateway Level Three, the final stage before the system goes live - when it was too late to make significant changes.

Despite the Gateway process being mandatory, the OGC still needs to be invited in by departments running projects, and doesn't have the power to take the initiative itself.

An Inland Revenue spokeswoman said: 'The Inland Revenue agreed with OGC that a review would be undertaken and focus on programme governance in its widest sense.

'As a result of the review, which found that the Programme was an exemplar of good programme management, the Revenue was able to move on to Gateway Four.'

According to a report this week by the Parliamentary Office of Science and Technology, the OGC considers Gateway Zero to have had particular impact on improving the success rate of government IT projects.

'It is compulsory and expected at the start of a programme and is recommended for high-risk projects,' says the report.

The tax credits system, implemented by EDS, was launched in April to coincide with the new tax year.

System downtime and slow response times created backlogs of applications, and chaos for families applying for the credit.