The trees may be imported with plant passports from European member states, but only from pest-free zones.
Fera say there are no pest-free areas in Europe yet declared.
In January, the UK issued a pre-notification notice on plane and sweet chestnut, aiming to keep the UK free of plane canker and chestnut blight. (Ceratocystis fimbriata f.sp. platani and Cryphonectria parasitica).
And in November, the UK issued further measures on Platanus and Castanea meaning movement of "trees in, around and out of England will need to be accompanied by official documentation confirming they are from an area free of any relevant disease".
In addition, the existing notification scheme for imports of certain tree species from EU member states was extended to include pine trees.
UK nurseries have raised the issue with Fera and the HTA.
HTA policy manager Gary Scroby said: "I don’t think the rules have been stated as clearly as they should have been.
"We’ve sent the rules to all our nursery members but that’s only as small proportion of the supply chain that imports trees.
"Landscape contractors might want six plane or sweet chestnut and get them direct from their continental supplier who is not aware of the UK national legislation and they will be brought in."
Scroby said imports should have dropped down to zero and "no-one should be shipping them in". He added: Their stocks will be dwindling if they are importers."
He said he has "no evidence" of systematic flouting of the rules but that he suspects trees will be arriving through direct-import pathways.
There are concerns suppliers from elsewhere in the EU may not want to go to the expense of paying for the surveys and inspections necessary to declare themselves free. The effective prohibition status of sweet chestnut and plane is now similar to the chalara situation.