The schoolgirl, who worked for three hours every weekend, was trained to operate an electrocardiograph monitor at Perth Royal Infirmary in Scotland.

Her duties involved attaching electronic terminals to patients' chests and taking readings to doctors.

Health chiefs at Perth and Kinross NHS trust at first defended hiring her - but she has now lost her job.

Jim Devine, senior Unison regional officer, said he planned to write to Scottish health minister Sam Galbraith asking for an immediate investigation.

"I am very concerned that a schoolgirl has been allowed to operate a crucial piece of equipment in something as important as a cardiovascular unit," said Mr Devine.

The Scottish Office said today: "While we are assured that there was no breach of employment requirements for school children, we cannot condone the involvement of such young people in patient-related duties.

"We are glad to see that the trust has reaffirmed that such duties will not be undertaken in this way in the future."

Dr William Gray, director of Perth and Kinross NHS trust, initially defended the hospital's decision to employ the girl, saying he had confidence in her ability to handle the post.

"The duties involved would be an inappropriate use of skilled nursing or medical staff, and the employment of someone else to do this work means that doctors and nurses are freed to undertake more appropriate clinical duties," he said.

"The individual concerned has received full training to carry out the job, which can be learned in a few hours."

But shortly afterwards the girl was dismissed by the trust's director of personnel, Bob Ironside.

He said: "Whilst these duties have been undertaken in a supervised environment and do not involve anything other than the recording of basic data, the trust recognises the sensitivities associated with this practice.

"It has been decided that such duties should no longer be undertaken by individuals in this age bracket."