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Drugs firm Aspen is to pay £8m to the NHS after an investigation into the supply of a life-saving drug to thousands of patients.

The Competition and Markets Authority (CMA) said the firm offered to make the payment following an investigation.

The CMA said it suspects competition law was broken after Aspen paid two rivals to stay out of the market.

This left Aspen as the only UK supplier of fludrocortisone and able to set prices without competition, it added.

Aspen may also have to pay an additional £2.1m fine if the CMA formally decides the company broke the law.

Andrea Coscelli, the CMA's chief executive, said it was "unacceptable for the NHS - and the taxpayers who fund it - to have to pay millions of pounds more than they should for this life-saving drug".

The prescription-only fludrocortisone is mainly used to treat Addison's Disease, a rare deficiency of the adrenal glands.

This is the first time a CMA investigation will secure a payment such as this for the NHS.

In addition, Aspen will ensure that there are at least two fludrocortisone suppliers in the UK.

Shares in the firm dropped nearly 7% after the announcement.

Aspen said in a statement to investors that commitments it made, including the £8m payment to the NHS, did not "constitute an admission of any wrongdoing by Aspen" under EU law.

However, it said it had admitted liability "for entering into an agreement to acquire a potential competitor in fludrocortisone with the consequence that the conclusion of this agreement resulted in anti-competitive behaviour".

It added that the CMA would impose a penalty "which shall not exceed £2,101,954".