Hundreds of important NHS hospital tests could be put on hold due to a worldwide shortage of a radioactive isotope, doctors have warned.
Three of the five reactors which produce molybdenum-99 have been shut down, severely reducing the amount available.
Specialists said urgent tests would have to be prioritised.
However, they are concerned that waiting time targets in England may interfere with their planning.
The problem is likely to continue until the end of September at least.
"It is likely to get worse before it gets better"
A wide variety of tests, such as heart and kidney function tests and bone scans (including those looking for tumours), involve the injecting of dyes which use molybdenum-99.
It is produced from uranium using a nuclear reactor and, as it has a half-life of less than three days, it cannot be stockpiled but has to be transported and used as soon as it is produced.
There are three commercial reactors in Europe which produce the isotope and all of them are currently closed - two for planned maintenance, and the other because of an unexpected problem.
Since last weekend, the NHS has been receiving approximately half its normal supply, and this is expected to dwindle further in the weeks to come.
Six week worry
Professor Alan Perkins, the honorary secretary of the Sydenham-based British Nuclear Medicine Society, said that while most tests could be delayed for a short time, others needed to be carried out within days and hospitals would have to prioritise patients accordingly.
In England, hospitals are expected to provide tests for patients inside six weeks and Prof Perkins said he feared that a non-urgent patient approaching that deadline might be given priority instead of one with an immediate need.
He said: "It is likely to get worse before it gets better. In some cases patients can be given a different kind of investigation, such as an MRI scan.
"However, patients will need to be prioritised in terms of clinical needs and we are concerned that in some cases there will be pressure to meet waiting targets - which could prevent this happening.
"Some people have already contacted us to say that this has been the outcome."
Other parts of the UK do not have the six-week testing target.
Urgent priority
This is not the first time that supplies have been hit by a reactor shutdown - a stoppage at a Canadian reactor last year delayed thousands of tests.
In the long term, Professor Perkins said, the UK should consider investing in a new reactor to help reduce its reliance on ageing facilities in other countries.
A spokesman for the Department of Health said it was aware of the problem and that hospitals would be taking steps to cope with it.
"The impact will vary from trust to trust, depending largely on the test being carried out, how urgent it is, and of course the flow of supply.
"Urgent patients will continue to be prioritised. Routine patients may need to wait a little longer until supply comes back on-line."
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