Image caption With helium in short supply, there are calls for its use in balloons to end

It is something guaranteed to catch the eye of most young children on a day out - a huge bunch of floating, brightly-coloured helium balloons for sale.

And for many people, a vital element in arranging a party is sitting down with a cylinder of helium to fill dozens of balloons with the lighter-than-air gas.

But according to one academic, such occasions may soon be a thing of the past.

Tom Welton, a professor of sustainable chemistry at Imperial College, London, believes that a global shortage of helium means it should be used more carefully.

Helium cools the large magnets inside MRI scanners - the medical devices that provide doctors with detailed images of what is happening inside their patients' bodies.

Helium uses Most of the world's helium is mined in the United States

It has the lowest boiling and melting points of the chemical elements

Helium is used to keep the large magnets in MRI scanners cool

It is also used in the space programme to clean rocket engines

Prof Welton told BBC Radio 4's Today programme: "We're not going to run out of helium tomorrow - but on the 30 to 50 year timescale we will have serious problems of having to shut things down if we don't do something in the mean time."

He added: "The reason that we can do MRI is we have very large, very cold magnets - and the reason we can have those is we have helium cooling them down.

"You're not going into an MRI scanner because you've got a sore toe - this is important stuff.

"When you see that we're literally just letting it float into the air, and then out into space inside those helium balloons, it's just hugely frustrating. It is absolutely the wrong use of helium."

If I thought this industry was taking helium away from the medical profession, I would be looking at doing things differently. John Lee, Balloon Association

Helium is extracted from deep underground, where deposits of the gas have built up.

It is usually mined as a by-product of natural gas extraction. But resources are finite and demand is increasing, which is why supplies are restricted.

Two years ago, the shortage of helium prompted the American Nobel Prize-winning scientist, Robert Richardson, to call for the price of one party balloon filled with the gas to cost more than £60.

"There is a current shortage," said Doug Thornton, chief executive of the British Compressed Gas Association - the body which represents commercial suppliers of helium and other gases.

"That has led to a two-year price-hike, although we expect that prices may drop again, as new reserves are found in places like Russia. But there aren't many alternatives in terms of supply."

Last month the UK's Balloon Association, which represents the party and promotional balloon industry, said prices were going up and supplies were under pressure.

It estimates that at present it costs between 30p and 50p to fill a single balloon with helium.

Image caption There are a finite number of helium atoms on the planet

But John Lee, the association's chairman insisted that the helium its members put into balloons, was not depriving the medical profession of the gas.

"The helium we use is not pure," he said. "It's recycled from the gas which is used in the medical industry, and mixed with air. We call it balloon gas rather than helium for that reason.

"There is no way the balloon and party industry would even consider taking badly-needed helium from the medical profession. That is important - people have to come first.

"If I thought this industry was taking helium away from the medical profession, I would be looking at doing things differently."