Monitor has revealed it is examining a potential loophole that could allow foundation trusts to escape the patient guarantees laid out in the NHS constitution.

The loophole has the potential to undermine the prime minister’s assurance that trusts would continue to be held to the NHS constitution, including waiting times pledges, which he gave after the NHS Future Forum reported in June.

The regulator’s board minutes revealed that directors considered “the ethical approach to decision making expected from the boards of foundation trusts, particularly regarding their duties in respect of the NHS constitution”.

According to the minutes: “Whilst foundation trusts were required to ‘have regard’ to the constitution in their decision making, a decision not to adhere to it would not necessarily mean a breach of their terms of authorisation.”

They added the issue would become “increasingly important” as trusts faced a harsher operating environment.

The meeting decided further work was required, to help Monitor make a “clear decision on the lines of accountability in such cases”.

The Foundation Trust Network said the annual operating framework set out how trusts should observe the constitution.

A spokeswoman said: “Not complying with the operating framework might be a breach of the terms of authorisation, but might not be. So Monitor is right to look at this on a case by case basis. I doubt any foundation trust would deliberately set out to ignore the provisions of the constitution.”

An acute foundation trust chief executive said the 18 week deadline within which patients had to start consultant-led treatment could be affected. “We are seeing primary care trusts really squeeze trusts on the amount they will spend on elective care – in effect putting a cap on it – which will inevitably result in 18 week breaches,” he said.

Another acute trust chief executive added that although boards would find it difficult to disregard the constitution, economic pressures could result in “slippage” against its provisions. “If reflected in commissioner contracts [any slippage could] be addressed through the contractual process,” he said.