Gaining foundation trust status is not yet driving sustained improvements in performance, the regulator Monitor has concluded in its review of their performance last year.
In a report presented to Parliament last week, Monitor urged foundation trusts to 'strive for continuous improvement in performance'.
It concluded that improvements seemed to stop once trusts had won foundation status.
'While foundation trusts are maintaining high levels of performance, we are not yet seeing improved performance after trusts secure foundation trust status,' it said.
'In other words, necessary improvements in governance and financial strength are currently driven by the assessment process and the pace of improvement is not generally sustained after authorisations.'
It added that, although the trusts had made good progress in important areas such as infection control, the regulator would be 'unequivocal in demanding consistently better results'.
Commenting on the review's findings, Monitor chair Bill Moyes said: 'While NHS foundation trusts have made a solid start, we are not yet seeing them fully exploit their freedoms to drive up performance.
'In part, this is a reflection of the maturity of foundation trusts as autonomous organisations and of the continuing uncertainty over commissioning intentions for future services.'
He called on foundation trusts to demonstrate that they could play a central role in delivering the service changes and improvements that would arise from junior health minister Lord Darzi's review of the NHS.
Mr Moyes said that foundation trusts would have to raise their performance if they were to meet the challenges ahead.
Monitor reported that despite their improving operational efficiency, foundation trusts did not outperform NHS trusts on measures of clinical productivity such as day case rates or length of stay.
It called for foundations to look at remodelling clinical pathways to improve efficiency and enable increased surpluses to be reinvested in improving patient care.