ONE of Europe's deepest mines has been ordered to improve safety after a blaze broke out more than 1,000 metres below the North Sea last month.

Boulby potash mine, in east Cleveland, has been issued with an "improvement notice" by the Health and Safety Executive (HSE) as it continues an investigation into the fire.

It started just after midnight on April 13, five miles out to sea and 1,100 under the sea bed, and was believed to be caused by polystyrene blocks catching fire.

The improvement notice is believed to be in relation to a risk assessment being carried out on the polystyrene.

Seven workers were taken to hospital suffering from smoke inhalation. More than 100 staff were underground at the time, and 11 were temporarily unaccounted for.

The HSE said it had served the improvement notice in relation to the fire, and that mine owners Cleveland Potash Limited had until June 27 to comply.

A spokeswoman said they had 21 days to appeal the decision, and that HSE was continuing its investigation of the underground fire, with a final report expected in a few weeks.

A statement from Cleveland Potash said: "We have been working closely with HM Mines Inspectorate in the investigation into the recent incident.

"In fact the investigation is still ongoing and is likely to continue for some time.

"Safety is always our first priority and we are fully committed to working with the Inspectorate in order to identify ways in which procedures can be improved.

"We will, of course comply with all the measures set out in the improvement notice.

Boulby employs about 1,100 people, but is cutting jobs as part of a new business strategy.