Paul Roberts said he was working to improve wards as soon as possible
Plymouth Hospitals NHS Trust has been told it must improve its hygiene and cleanliness standards or risk a possible fine or legal action.
The trust, which runs the city's Derriford Hospital, was one of eight in the UK to be marked out as particularly poor by the Care Quality Commission.
The trust has been told to improve hygiene standards on five of its wards.
Its chief executive, Paul Roberts, said he was "disappointed" but said they were working to improve cleanliness.
We need to make sure that we give confidence to the people that use this hospital
Paul Roberts, chief executive Plymouth Hospitals NHS TRust
Derriford was awarded conditional registration after falling short of the required standards on Hound Ward, Bracken Ward, Honeyford Ward, Braunton Ward and in the Medical Admissions Unit.
A spokesman for the Care Quality Commission (CQC) said the hospital was expected to clean-up the wards "immediately".
Members of the Healthcare Commission, whose responsibilities have now been taken over by the CQC visited Derriford in July 2008 and demanded improvements.
Although "progress was made" at the time, the spokesman said, another surprise inspection in February revealed "recurring problems in some wards."
Legal requirement
Mr Roberts said: "While I'm disappointed, I recognise that cleanliness is a public concern and we need to make sure that we give confidence to the people that use this hospital."
He said the "whole team" at Derriford was determined to deal with cleanliness issues to get full registration as soon as possible.
All NHS trusts who provide direct healthcare are now legally required qualify for registration with the Quality Care Commission, which launched on 1 April.
To do so they must meet government regulations on cleanliness and infection control.
In all, 388 NHS trusts have been included on the register, 21 were given conditional registration and eight of those were told they had failed to reach the required standards.
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