MPs have criticised the Department for Work and Pensions for a series of rule breaches in which official statistics were used inaccurately, inappropriately, or to "spin" stories about benefit claimants.

The Commons work and pensions committee also criticised the DWP for shortcomings in the management of claims for Personal Independence Payments (PIP), a disability benefit that replaces the Disability Living Allowance, saying it was unacceptable claimants were having to wait six months or more to find out if they were eligible.

A report by the MPs warned the DWP to exercise care in the language used in its press releases and ministerial comments to ensure they do not feed into "negative preconceptions and prejudices about people on benefits".

It cites examples in the past few months where the UK Statistics Authority (UKSA) criticised the use of DWP statistics, including by the secretary of state, Iain Duncan Smith, and Conservative Party chairman Grant Shapps.

Dame Anne Begg MP, the committee chair, said: "Statistics should be used to shed light on policy implementation, not to prop up established views or feed preconceptions.

"Government efforts to promote a positive image of disabled people will be undermined if the language used by DWP when communicating benefit statistics to the media feeds into negative perceptions and prejudices about benefit recipients, including disabled people."

The committee said it had warned the government as early as 2011 to take more care over the way it presented information on benefits statistics to the media. Ministers had replied then by saying they had a "robust" system in place to ensure no abuses took place.

However, the committee notes in a report into DWP performance, published on Tuesday , that problems still remained and that the UKSA had reprimanded the department a number of times in 2013 for the way it handled welfare statistics.

In one case, the Conservative party had put out a press release which quoted party chair Grant Shapps citing DWP figures that purported to show nearly a million people had dropped their incapacity benefit claim rather than face a work capability test. The UKSA found that two sets of figures had been erroneously and misleadingly conflated.

Duncan Smith said he and his officials had not prepared one criticised Conservative party release, and he had had "conversations" with Shapps to ensure in future he checked with the department if he was going to say something about DWP statistics.

In a separate case, Duncan Smith was officially reprimanded for claiming that the threat of the benefit cap had directly persuaded 8,000 of claimants to get a job. This clearly demonstrated that the cap was working, he said. But the UKSA ruled that there was no statistical evidence to support this.

The DWP director of communications John Shield told MPs that Duncan Smith had the right to make clear his "opinion" on "what he thinks the data are saying". But he admitted that on this occasion the DWP press office had been involved in the preparation of the secretary of state's claims.

The committee report says government statistics should be presented fairly, accurately and unspun, "and this is especially the case when they are being used to justify a particular policy or a particular allocation of resources."

Regarding delays in PIP, the report urges ministers to involve financial penalty clauses to force private contractors Atos and Capita to speed up the claims process.

Begg said: "Many disabled or sick people face waits of six months or more for a decision on their PIP eligibility. Even those with terminal illnesses are having to wait far longer than was anticipated. This not only leaves people facing financial difficulties whilst they await a decision, but causes severe stress and uncertainty. It is completely unacceptable."

The committee's findings echo a recent National Audit Office report, which concluded that the PIP programme suffered from "poor early operational performance" leading to long and uncertain delays for claimants.

A DWP spokesman said: "PIP is a completely new benefit with a new face-to-face assessment and regular reviews. In some cases this end-to-end claims process is taking longer than the old system of Disability Living Allowance, which relied on a self-assessment form.

"We are working with providers to ensure that all the steps in the process are as smooth as they can be and the benefit is backdated so no-one is left out of pocket.

"Claims for terminally ill people are fast-tracked and Macmillan has acknowledged that improvements in the system have already been made. Latest statistics show over 99% of people with terminal illnesses who have applied have been awarded the benefit, which means over 9,500 terminally ill claimants are now receiving PIP."