Patricia Hewitt's plans to save £250m on NHS administration are ill-judged and a false economy, the Commons' health select committee says today in a unanimous report, approved by its usually loyal Labour majority.

It said the health secretary planned to merge primary care trusts - the lower tier of NHS management in England, responsible for spending 80% of the health service's £76bn budget. The upheaval followed a reorganisation, three years ago, when PCTs were established. It usually took three years for the benefits of any reform to be realised and the distraction caused by the latest proposals was "ill-judged in the extreme," the MPs said.

"The cycle of perpetual change is not conducive to the successful provision and improvement of health services. Major restructuring should only be undertaken if there is an overwhelming argument in its favour; in this case there is not."

The MPs' intervention came as Ms Hewitt was preparing a presentation to the Cabinet tomorrow on the next stage of reforms, to increase competition among GPs and other providers of health and social care outside hospitals.

The report may serve as a warning that she cannot rely on support from Labour backbenchers unless she prepares the ground more carefully than she did on PCT reform and a partial smoking ban.

The committee said there was little evidence the costs of reforming the PCTs would be outweighed by the benefits. The plan was announced in July after parliament broke for the summer giving the NHS 11 weeks to respond. Such brief consultation was "insufficient and flawed".