The dredging was carried out to aid access to Mostyn Docks

Van Oord Ltd and Volker Stevin Ltd were employed by the Port of Mostyn to carry out dredging, Mold magistrates' court heard.

But they dumped dredging material without proper licences. Both firms pleaded guilty to the charges.

Charges against the Port of Mostyn and manager Jim O'Toole were adjourned.

They face charges of causing or permitting the unlicensed deposit of articles, which they deny.

Their case was adjourned until 12 August.

The court heard a deep channel in the estuary was being dredged for larger vessels entering Mostyn Docks in 2001 but the companies had ended up with double the amount of material they expected.

It was dumped in Mostyn Deep, causing complaints from fishermen.

The amount deposited was more than 819,000 cubic metres, described as equivalent to 15 football pitches 10 metres in height, the court was told.

The material protruded above the water level at one stage.

Van Oord ACZ Ltd, of Rolls House, Fetter Lane, London, admitted two charges of depositing dredging materials on the sea bed without a Food and the Environment Protection Act Licence.

They were fined £20,000 for each offence and £646 costs.

Volker Stevin Ltd admitted depositing rock armour without a licence. They were fined £20,000 with £323 costs.