NHS services for children are struggling to cope, partly due to female doctors going on maternity leave and working part-time, experts have warned.
A survey of clinical directors for the Royal College of Paediatrics and Child Health (RCPCH) found worries over patient safety and concerns that units could close due to too few staff.
Some 78% said they were concerned about how the service will cope in the next six months while others said the wellbeing of trainee doctors was at risk.
Expensive locum doctors are being used to plug gaps in paediatric rotas, with up to 63% of some shifts filled by locums, the survey found.
A total of 157 clinical directors or deputies from 214 units across the UK responded to the poll.
Dr Simon Clark, workforce officer at the RCPCH, said: "Doctors choosing paediatrics as a career are likely to be family focused, which is an important attribute for their professional role.
"We see that many are choosing to start their own families sooner within the postgraduate paediatric training programmes.
"And as around 75% of doctors in paediatric training programmes are female, there are a high proportion of doctors on maternity leave.
"There are also a high proportion of doctors choosing to work less than full-time once they have their own children."
Dr Clark said services were at risk unless action was taken to plug gaps in the workforce.
Overall, there has been an increase in the vacancy rate from 10.5% in December 2012 to 12.1% in December 2014.
He said: "If we don't have safe staff numbers to deliver services, then there is a very real threat that we could see units closing their doors," he said.
The RCPCH also pointed to changes in immigration rules which prevent doctors working in the UK unless they earn £30,000 a year within four years of employment.
It called for an expansion in the number of qualified nurses able to deliver care, more training for GPs, and looking to entire regions to plug rota gaps.
Dr Clark said: "In the short term, collaboration is going to be key. The NHS cannot afford to be filling rotas with locums and it is not safe to rely on doctors who have already worked long shifts to plug gaps - trusts must utilise the wider workforce."