It is understood to be the first time an entire oil rig has been shut down in the North Sea because of allegedly poor working conditions.

Saga Petroleum told Noble Drilling of Aberdeen temporarily to cease drilling operations on the Noble Al White rig.

Saga, based at Sandvika in Norway, said the decision was made after an inspection and reports from Norwegian trade unions that workers ate and slept in unhygienic quarters and were subjected to sexual harassment and racist treatment.

One worker is quoted in the Oslo Dagbladet newspaper criticising the conditions on board.

He said: "I've worked in the North Sea for 20 years, but I've never seen anything like it. The rig is like a pigsty and I'm worried about my health."

He also claimed the rig's ventilation system was faulty, including areas where hazardous chemicals are stored, and that bathrooms and toilets were not properly cleaned.

He said a worker had been bullied daily by his boss, and that Portuguese workers were subjected to racial discrimination. Female catering staff suffered sexual harassment, Mr Boe added.

Saga said the rig would be shut down as soon as possible when it is safe to do so and added: "Saga takes the reported assertions very seriously and is now working to throw full light on the conditions on board.

"This means that one will be looking at the drilling contractor's observance of regulations and contractual requirements and on Saga's own follow-up of contractors."

An Aberdeen spokesman for Texas-based Noble Drilling said the 100 crew members - Norwegians, Portuguese and Britons - would stay on board while Saga carries out its investigation.

He said: "These allegations are taken very seriously irrespective of the issues being factual or not and Noble Drilling will re-emphasise its procedures and policies and have taken action accordingly.

"In the process, Noble Drilling will work closely with all parties concerned."

He added that "constructive and open discussions" had taken place with Norwegian authorities and Saga Petroleum.