New controls to curb the National Health Service's agency staff bills have saved £600 million in the first year of their introduction, but spending is still too high, says health service regulator NHS Improvement.

Measures to reduce the money spent on agency workers, and encourage staff back into full time employment, were brought in in October last year, after it emerged that the NHS spent over £3 billion on agency staff in 2014/15.

Now almost three quarters of trusts (73 percent) have successfully reduced their agency spend, and over half of these have cut spend by more than a quarter.

However, given that the sector is still spending some £250 million a month on agencies, NHS Improvement is introducing a new stream of measures to further cut the amount being spent on agency staff.

These include: publication of league tables of agency spend on best and worst performing trusts; collecting anonymised information on the 20 highest earning agency staff, per trust, and of long-standing agency staff; and additional reporting to NHS Improvement, including on shifts that cost over £120 an hour.

"The NHS simply doesn't have the money to keep forking out for hugely expensive agency staff," said Jim Mackey, the group's chief executive.

"Over a third of the agency bill is spent on medical agency fees, so we need to ensure agencies and doctors do their bit to make sure they're not overcharging. With hospitals across England struggling with patient demand, trusts need all the support they can get and trying to cash in on the NHS just isn't on."