Maternity services at Watford General Hospital must improve, an NHS watchdog has warned, after it received the worst possible rating.

But West Hertfordshire Hospitals NHS Trust, which runs the birth unit, hampered its own review by failing to provide complete data, which meant four categories automatically received the lowest score.

It was, however, recently rated as one of the safest maternity units in the country with a mortality rate of less than one in one thousand, compared to one in 300 nationwide.

Watford MP Claire Ward will return to the unit next month for the birth of her second child, following the arrival of her daughter Ruby at the same hospital 18 months ago.

Due to the data failure, West Herts was rated as one of the "least well performing" in the country, with an overall rating of 1.958 out of five across three areas - clinical focus, women centred care, and efficiency and capability.

An "acceptable" score of three out of five was given to sections including maternal morbidity, which charts the number of good outcomes from deliveries, choice of antenatal care and choice of delivery method.

However, a score of one was handed down for staff training and experience levels, the number of women who wanted and had antenatal classes, and staffing levels.

It is unknown which areas were affected by incomplete data.

Jan Filochowski, the trust's chief executive, said: "We can and must do better.

"We have already taken action to ensure all women have a positive experience".

In particular, 18 new midwives have been hired, multidisciplinary training is in place and the trust now provides a wide range of education programmes for parents.

Newly qualified midwives will be "rotated" to gain more experience in the community before returning to hospital, and nursery nurses will work in post-natal wards to help new mothers and allow midwives to provide one-on-one care for women in labour.

The trust also plans to increase the number of midwives and consultants, increase specialist support for newly qualified midwives, update information for women and their partners, and reduce the rate of caesarean sections.

Professor Graham Ramsay, medical director and director of patient safety, said: "One of our top priorities remains recruiting more midwives.

"Another priority for the trust is to improve our data capture.

"If we had been able to provide the data for all categories we would have scored higher."

Ms Ward described the maternity unit as Watford General's "jewel in the crown".

She said: "I don't think it reflects what goes on in Watford.

I think the maternity unit is one of the better services. People should feel reassured.

"I have every confidence in it and I would not hesitate recommending it to other mums."