Nicola Sturgeon has called for an official audit of figures

The SNP said 17,500 patients have been waiting more than nine months for treatment and have called for a probe but ministers have rejected the claim.

The row follows a BBC Scotland report about tens of thousands of patients who do not qualify for treatment within the current "guarantee" of nine months.

Audit Scotland said it has been carrying out a review of waiting lists.

Patients are given what is called an Availability Status Code, or ASC, when they cannot attend an appointment for personal, social or medical reasons.

It is the people in this category which the SNP has decided to highlight and their wait for treatment which lies behind its calls for an official inquiry.

These people should not be cast into the wilderness and left with no guarantee whatsoever of when they're going to have treatment Nicola Sturgeon SNP

Earlier this month, BBC Scotland revealed the number of people with an ASC was at record levels.

The SNP said it has figures which show that there are now more than 35,000 people who fall into the category.

Speaking to BBC Radio Scotland, the party's deputy leader Nicola Sturgeon said half had waited more than nine months for treatment and 12,500 had been waiting for more than a year.

Ms Sturgeon said: "Every quarter, or certainly in recent quarters, the health minister has said that nobody in Scotland waits more than nine months for hospital treatment.

"So at a very basic level the information that the government is giving out is simply not true.

"Only 10% have wilfully not turned up for their treatment, the rest have fallen into that category.

The SNP says 17,500 are waiting too long for NHS treatment

"Most of them are there because for some personal reason or another they've not managed to keep their hospital appointment.

"These people should not be thereafter cast into the wilderness and left with no guarantee whatsoever of when they're going to have treatment.

"The number of people in this excluded category has gone up quite considerably over the last year and I think that says that there is now a reason to look at this very closely and I would like Audit Scotland to look at this to make sure that there is nothing untoward happening here."

However, the executive denied it had hidden the figures and said 90% of the delays were "patient driven".

It also said there was no secrecy about the existence and application of ASCs.

A spokesman said the figures were available on the government's Information and Statistics Division (ISD Scotland) website (ISD Scotland).

"There is no secrecy about ASCs and how they are applied," he explained.

There is no secrecy about ASCs and how they are applied Scottish Executive

"We have already announced that we are abolishing them by 2007 to make the whole system clearer, more consistent and fairer to patients.

"At present, around nine out of 10 ASCs are patient driven - patients who cannot attend for personal or social reasons, who did not turn up, or who have a medical condition which prevents treatment."

Audit Scotland said its latest review of waiting lists is due to be published soon.

A spokesman said: "In 2002 we drew attention to the issues involved in managing hospital waiting times, including the use of deferred lists, and we recommended they be reviewed and this recommendation was accepted by the health department.