The NHS in England is facing problems recruiting locum hospital doctors, a leaked government memo shows.
Locum positions are often filled by junior doctors
The document, obtained by the Health Service Journal, said that some trusts were even having difficulties getting applicants to attend interviews.
The problem seems to have been caused by changes in junior doctor training as young medics are generally used to fill the locum posts.
The government said it took the issue seriously.
Locums work across a variety of specialities and are used to fill posts until a permanent appointment can be made.
Junior doctors often recruited to carry out the duties in-between their official training positions.
But a reform of medical training meant the end of staggered start dates for the training posts, meaning there is a shortage of doctors in the system.
The memo was sent by a Department of Health official to health bosses asking for evidence of problems.
The official said: "Anecdotally we are aware that service in some trusts may be experiencing problems in finding locums."
The replies received show a range of difficulties across the country.
One reply described the problem as "extensive".
It said: "We are having increasing difficulty filling any and all vacancies and locum requests."
One hospital manager said: "We are lucky if the applicants attend for interview, and on a number of occasions there have been no applicants to interview on the day."
NHS North East reported more than 42 doctors' vacancies for which they are unable to find any cover.
And another hospital chief said locums "have proved difficult if not impossible to find" in some specialisms, including plastic surgery.
A Department of Health spokesman said: "This problem has been circulating anecdotally for some time.
"We do take it seriously, which is why we asked SHAs to help us understand its extent and nature at the beginning of February.
"It is worth bearing in mind that the NHS employs around 120,000 doctors in England and, whilst some Trusts have reported issues, many haven't.
"There have always been some problems staffing some rotas in shortage specialties."
Ram Moorthy, chairman of the BMA junior doctors committee, said: "The BMA has had reports from junior doctors across the country that rotas are operating below full strength.
"They're being pressured to work extra hours, often unpaid, to fill the gaps, and consultants are also taking on additional workload."