FEARS have been raised by the Government watchdog that more patients' treatment may have been affected by a mysterious drop in success rates at Scotland's largest NHS fertility clinic.

The Human Fertilisation and Embryology Authority (HFEA) wants the Glasgow Royal Infirmary centre to ask an independent expert to examine its results for a period last summer.

Doctors stopped treating patients at the GRI's assisted conception service in November after they noticed a dramatic drop in fertilisation rates. It is understood conception rates fell from around 30% to about 15%.

Staff carried out an internal review and ruled out clinical error.

They think dust from nearby building works contaminated the delicate artificial insemination process where sperm fertilises an egg in a Petri dish.

The HFEA's official incident report into the crisis reveals that 42 patients treated between September 1 and November 8 last year were affected.

But, in the document the HFEA also raised concerns that the issue may have had an impact upon some patients' treatment from May to July last year.

No figures about the number of patients possibly affected were given in the document and discussions are ongoing between the clinic and the HFEA, the state regulator which monitors, inspects and licenses IVF units.

In correspondence, the clinic insisted the results for May to July were consistent with its fluctuations.

It pointed out that other factors were responsible for the results. A number of women with "a poor prognosis" received treatment and doctors began replacing one embryo, rather than two, in order to reduce its twin pregnancy rate – as required by the HFEA.

The HFEA's report was written for the body's licensing committee on March 28. "The committee is asked to review the evidence provided by the person responsible [at the clinic] and consider whether to recommend that the centre should seek to have the evidence reviewed by an independent expert," it said.

Juliet Le Page, 54, of Edinburgh, who runs Fertility Concerns, which advises women trying to get pregnant, and who has two children after having IVF, said: "I really think the clinic should ask an independent expert to review the results from May to July. If the HFEA has raised this as an issue, then the clinic should act on it."

Jackson Carlaw MSP, Tory health spokesman, said: "Obviously this does raise considerable concerns about the clinic at the Glasgow Royal Infirmary.

"Alex Neil needs to investigate this matter urgently."

A £2 million refurbishment to the GRI's fertility service's laboratory is now under way.

A spokeswoman for the HFEA said discussions with the clinic were ongoing regarding whether an independent expert should examine the May to July results.

NHS Greater Glasgow and Clyde would not say how many patients may have been affected between May and July last year.

"It would not be appropriate to speculate until our discussions with HFEA are concluded," said a spokeswoman.

"We are still in dialogue with HFEA and the result of these discussions have not yet concluded, however the outcome will be made public."

Regarding the ban on treating patients, she added: "As has been well reported, the unit experienced a reduction in its usual success rates at the end of 2012 and as such we voluntarily transferred activity to the Nuffield Hospital in Glasgow while an investigation was carried out into the possible decline.

"The condition that HFEA has placed on the unit simply formalises the voluntary actions that we have already taken and indeed HFEA, who we are in regular dialogue with, have confirmed that they are fully supportive of the steps we have taken."