NHS trusts have been spending more than £1 billion a year on agency nurses due to poor planning, an influential group of MPs warned.
The bill grew by 40% in the five years to 2005 - despite the number of permanent nurses in the health service rising by a fifth over the same period.
The use of agency staff may also be compromising safety because 39% have not received basic life support training, according to the Public Accounts Committee.
Chairman Edward Leigh said temporary staff could be useful for dealing with short-term peaks in demand and holidays.
However, spending had been running at "worryingly" high levels despite the financial crisis in the NHS.
"Most trusts are failing to plan for and manage their demand for temporary nursing staff," he said.
"Too many do not have the management information they need to employ temporary staff cost-effectively. And too many are relying on temporary nurses to mask their inability to manage their permanent staff properly.
The Tory MP for Gainsborough insisted the problem was not simply wasting tax-payers' money, but could be putting lives at risk.
"At least 39% of temporary staff are not receiving supposedly mandatory basic life support training."
