The Irish-owned firm - which grows, sells and distributes mushrooms packed in Langford, north Somerset - was fined by Bristol magistrates on 17 December after pleading guilty to failing to register, recover and recycle waste packaging under the Producer Responsibility Obligations (Packaging Waste) Regulations 2005.
Under the new regulations, any company with an annual turnover in excess of £2m and handling more than 50 tonnes of packaging is required to register an approved waste packaging scheme. The court heard that by failing to register and pay fees, Monagham Mushrooms had avoided paying about £4,000.
The company was fined £4,000 with £2,000 costs. It was also ordered to pay £768 compensation in respect of an unpaid registration fee.
After the case an Environment Agency representative said: "This is a large business with a multi-million-pound turnover that should be aware of its legal responsibilities. By failing to comply it had an unfair financial advantage over its legitimate competitors."
Monaghan Mushrooms was unavailable for comment at time of going to press.