The “national roll-out” of Universal Credit will now only see the new system running at a handful of JobCentres across the UK this year, ministers confirmed.

Iain Duncan Smith, the Work and Pensions Secretary, insisted that Universal Credit remains on track and will fully operational by 2017. A more gradual timetable means the reform will be delivered “safely,” he said.

Universal Credit will merge several individual benefits for millions of working-age people into one payment. The sum paid to claimants will then be automatically adjusted to ensure that claimants are always better off in work than on welfare.

The new system requires the creation of two highly complex computer systems, one run by Mr Duncan Smith’s Department of Work and Pensions, the other by HM Revenue and Customs.

The scale and complexity of the new system has led to repeated warnings from Whitehall officials about potential problems and delays.

Those warnings have forced ministers to step back from bold promises about the timetable for delivering the new system.

In 2011, the Coalition said that from October this year, “all new claims for out-of-work support would be treated as claims to universal credit”.

But the DWP has now confirmed that the “national” roll-out will mean that just six JobCentres will offer Universal Credit from October.

Those sites – in Hammersmith, Rugby, Inverness, Harrogate, Bath and Shotton – will be followed by others in the following months, the department said.

Facing doubts about the reform, Mr Duncan Smith brought in Whitehall trouble-shooters to revise the Universal Credit plan.

That has resulted in a new plan for a much more gradual timetable for implementation. A “pathfinder” project for Universal Credit launched in April saw the system in use on just two JobCentres.

After questions about the computer systems involved in the reform, the DWP has also asked experts from the Government Digital Service to “explore enhancing the IT for Universal Credit”.

Liam Byrne, the Labour shadow work and pensions secretary, said that the plan for October raises questions about the entire welfare reform plan.

“Today we have final confirmation that the welfare revolution we were promised has collapsed,” Mr Byrne said. “Iain Duncan Smith must now ask himself if he is fit for purpose.”

The DWP insisted that the overall deadline for full implementation of Universal Credit by 2017 remains unchanged and will be met.

Government sources said that with a project of such size and complexity, some problems were inevitable.

It is better to address those problems as they arise than stick to “artificial” deadlines, Mr Duncan Smith told MPs.

“While we press ahead with delivery, we are also ensuring that we have the best long-term approach in place for this transformative benefit,” he said.

“I'm determined to get this right and will not follow the old ways of governing - launching with a big bang and having to clear up the mess afterwards. I will bring in this radical reform safely, and I'm committed to doing it by 2017 and to budget."