More patients at hospitals in Wakefield, Pontefract and Dewsbury died last year than would be expected.

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That has put Mid Yorkshire Hospitals NHS Trust in the bottom 12 hospital trusts nationally for the safety of patients.

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Fewer people treated at Leeds hospitals died than would be expected.

The Hospital Guide, published by independent firm Dr Foster, produces an annual report looking at different standards to assess patient safety. Looking at 13 indicators, it gives hospitals a score out of 100 to measure how compliant they are with patient safety measures.

Mid Yorkshire hospitals scored 6.16 – the second lowest in the region – mainly because of their mortality rates.

These compare how many patients are likely to die, taking into account things like whether they had several health problems, against how many deaths there actually were.

Hospital heads at Mid Yorkshire Trust say they have scored badly because Dr Foster revised the way it calculated the death rates and the figures failed to show show it was now better than average.

Julia Squire, chief executive of the trust, said: "Changes in the way Dr Foster calculate and standardise mortality rates has led to a number of trusts' mortality rates increasing, including Mid Yorkshire's.

"This is despite those trusts having made improvements in patient care and safety."

She said that until recently the death rates measured by Dr Foster had been around the national average but after the changes, the same data now showed more than expected.

"People can be confident about the care we offer and our focus on patient safety," she added. "I am very happy for my family to be cared for at our hospitals and I can recommend our services to local people."

Leeds Teaching Hospitals NHS Trust did well in the overall patient safety rating, achieving 83 out of 100.

However, it did badly on treating patients with hip fractures within two days of admission, as in NHS guidelines.

A spokesman for the trust said weekly review meetings had been started to speed up the process.

Airedale NHS Trust, based near Keighley, was named Small Hospital Trust of the Year after achieving a top rating for patient safety.