The world's largest security printer will not comment on speculation that a huge contract with the Reserve Bank of India is potentially at risk due to the production irregularities that it first revealed in July this year.

De La Rue has also been criticised for the opaque nature of the statement that accompanied news of Hussey's departure, in which it said it was still assessing the financial and operational implications of the irregularities at its Overton mill, in Hampshire, described as being "of a serious nature".

Despite this, the group said it would be unable to clarify the impact in the near future, causing consternation in the market. Its share price fell by 12% on the news to £6.93 and had only recovered marginally to £7.00 at the time of writing (52 week high: £10.21).

"If there's one thing the market hates it's uncertainty, and this has uncertainty stamped all over it," said Paul Jones, an analyst at Panmure Gordon. "The news could get worse."

Hussey, a 25-year veteran at the company, only became chief executive 18 months ago, doubling his total pay to £451,000 in the process. His responsibilities have now been taken over by chairman Nicholas Brookes and finance director Colin Child, who is himself a newcomer to the business, having only joined the group in June.

The loss of someone with Hussey's depth of knowledge is viewed as a major blow. "He's going to be a hard act to follow, you can't just waltz in and start talking the language that these guys at central banks talk – you've got to have somebody with expertise in banknotes and the high security market and in developing economies," said a source close to the company.

A recent report emanating from HT Media, owner of the Hindustan Times, said that India imported 95% of its banknote paper requirements from a raft of specialist suppliers, including De La Rue, although it stated that plans are afoot to build a specialist paper mill to increase its own production.

It added that, last year, the country imported 17,936 tonnes of this paper, costing Rs 1,173 crore (£161.2m).

Currency, which includes banknote paper production, is De La Rue's biggest business unit, generating around £410m of the total group sales of £561.1m, as well as an operating profit of £95.3m (group operating profit: £109.2m).