Work and pensions secretary says targets for moving claimants on to UC now altered, but insists policy is on time and on budget

The work and pensions secretary, Iain Duncan Smith, insisted on Monday that his flagship welfare scheme would be delivered on time and within its £2bn budget despite IT problems. But he admitted that the original plans to introduce universal credit had had to be altered.

The Office of Budget Responsibility said last week only 400,000 claimants of the 4.6 million that had originally been proposed would be on UC by 2015-16, but Duncan Smith said all 6.5 million people who will be required to work as a condition of receiving benefit will be on UC by the end of 2017.

Roughly 700,000 claimants not on the work-related activity aspect of the employment support allowance – a benefit for the most severely disabled – will not be moved on to UC, but Duncan Smith said he had never been specific about forecasting numbers. "We never really wanted to dwell on figures because you want to move and change it. But yes, I do accept this plan is different from the original plan."

Speaking on BBC Radio 4's Today programme, he also implied that he intervened when he first realised there was a problem with the IT systems for UC at the end of 2011, suggesting that he may not have been given a complete picture by civil servants on the progress of the project. He said he had brought in a new team a year and a half ago because he was concerned "the security and online aspects" were not going to work.

He said: "Let us get this clear: I introduced the 'red team' of outside experts to look at the plan back at the end of 2011 for the very simple reason that I was concerned we may end up repeating the mistakes that were the last government's over tax credits and the health departments. We are not going to do that because the plan has been reset."

He added: "The important thing here is that a government intervened to make a programme right before people are affected."

He also said he was working in tandem with the Cabinet Office's government digital service (GDS), and denied there was any conflict with the Cabinet Office. He said the GDS was "advising on the enhanced, fully rolled-out offering" but added that his department ultimately needed to take control of the IT system. He said the Cabinet Office IT system would eventually mesh together with the one developed by the DWP.

Duncan Smith had released figures on the morning of the autumn statement that revealed officially for the first time that the project was off the track. He is due to face detailed questioning on Monday afternoon by the work and pensions select committee.

Chaired by Dame Ann Begg, the committee has repeatedly raised questions about the effectiveness of the IT systems, and been given reassurances. Relations between the committee and Duncan Smith have become increasingly tense owing to what some of its members regard as prevarication at best and misleading answers at worst.

The shadow work and pensions secretary, Rachel Reeves, described the scheme as a shambles and accused Duncan Smith of failing to show leadership, pointing out that Labour had suggested an all-party review of the biggest change to the welfare system since its inception.