New rules to limit spending on agency workers were breached 540 times in just one week at Calderdale and Huddersfield hospitals.
Almost £20m was spent on agency doctors and nurses at the hospitals in the last financial year amid an NHS-wide staffing crisis.
NHS trusts around the country have been struggling to recruit enough permanent staff, leading to the rules on agency spend being routinely broken.
At Calderdale and Huddersfield NHS Foundation Trust, agency spending has helped build up a financial deficit of £20m.
A report to the trust board said agency spend was “the biggest financial challenge for next year.”
There were 540 breaches of the agency cap in the week of March 28.
The report added: “This shows the scale of the challenge.”
Health regulator Monitor has set the trust a £14.95m agency spending cap for 2016-17.
Helen Barker, chief operating officer at Calderdale and Huddersfield, said: “We are doing everything we can to reduce the amount we spend on agency staff both through our ongoing recruitment drive and through senior level review of our use of agency staffing.”