The Agriculture and Horticulture Development Board (AHDB) is consulting on changing the levy regime. The proposals include reducing the levy threshold from £60,000 to £20,000, ending the current system of permissible deductions and reducing the levy rate from 0.5 per cent to 0.35 per cent of turnover.

Although the proposals are not expected to increase the levy that the HDC collects overall, it could mean nurseries that retail end up paying more.

This is because the turnover figure is adjusted to take into account the cost of packing materials, costs incurred transporting materials to customers and the purchase cost of goods bought in to be grown-on.

According to HTA business development director Tim Briercliffe, the reduction in the levy rate would not balance out the extra costs if nurseries were not allowed to deduct those outlays.

"As a nursery you have got pots, labels and packaging. A lot of value goes into those," he said. "Nurseries also do a lot of trading with other nurseries abroad and in the UK, which makes it unfair because the levy will have already been paid by the first nursery so it is double counting."

The consultation follows an independent study by Cranfield University economist Sean Rickard. The HDC commissioned the report following dissatisfaction from some apple and pear growers that levy charge increases had arisen from the change made to the method of calculation from an areato a turnover-based reckoning.

British Protected Ornamentals Association chair Sarah Fairhurst said she was concerned that collecting from smaller growers would "add to the administrative burden" for the HDC.

HDC director Bill Parker said: "We don't want to get into a law of diminishing returns. It's not a done deal."

Responses to the consultation can be sent to the AHDB until 24 February. See consultations at www.ahdb.org.uk.