Foundation trusts could see one of their freedoms curtailed if they continue to make 'overly optimistic' predictions about their performance.
Monitor has warned they will face independent investigations if performance predictions do not turn out to be true.
Reviews of the self-certification process, in which trusts set out how well they think they perform against a range of targets, will be carried out if boards cannot demonstrate they have robust systems in place to ensure accuracy.
In its report on trusts' annual plans for 2007-08, the regulator said some trusts had registered compliance with targets only for it to later emerge that the systems used to report this were inadequate. Information had also been inconsistent, inaccurate and not reported on time.
Monitor chair Bill Moyes said he suspected the nine out of 70 trusts that predicted they would not hit all targets this year was less than the true number.
For example, eight pledged at the start of the year to hit a target to reduce MRSA but more than that were failing to meet this standard by the end of June. In the last quarter of 2006-07, 34 out of the then 59 foundation trusts were missing this target.
Mr Moyes told HSJ: 'We are always prepared to be pleasantly surprised. But we don't like people being overly optimistic and then finding they can't deliver.'
If potential problems were not on boards' agendas, it would make performance more difficult to manage later in the year, he said.
How well trusts were performing by the end of next month, halfway through the operating year, would help Monitor decide whether to begin the reviews.
Mr Moyes acknowledged the news 'might not go down well' with foundation trusts. But he said: 'Most people realise that if a system of self-certification does not deliver an accurate picture the regulator has a responsibility to find out why.'
But Foundation Trust Network director Sue Slipman said the idea could be interpreted as a creep towards performance management.
'It looks as if Monitor is going to cease to be a risk regulator and become a performance manager and that is where we would have a problem,' she said.
University College London Hospitals foundation trust chief executive Robert Naylor said the proposal from Monitor was 'perfectly legitimate'.
'If Monitor has concerns that boards are not taking their responsibilities seriously enough then I would expect them to want an independent review of that.'
As of 31 July foundation trusts had a combined surplus of£198m. But Mr Moyes said there was likely to be more money in trusts' hands because primary care trusts have not drawn up long-term commissioning plans.