Exporters of plants and flowers from mainland Europe are turning their backs on supplying Britain as “painful” new Brexit border checks are putting some trading relationships at “breaking point”, garden centres and nurseries have warned.

The Horticultural Trades Association (HTA), which represents garden retailers and growers, said long-held links between British nurseries and EU suppliers were now being put under strain because of the delays and costs associated with the new border processes.

The comments were included in a letter to Lady Hayman, the borders, biosecurity and plant health minister, sent by the HTA on Friday, in which it said that some specialist transporters were now withdrawing from the UK market completely.

On 30 April the government introduced new checks of plant and animal products entering the UK from the EU.

The moves were brought in at selected border posts at ports to enhance Britain’s biosecurity and prevent harmful plant and animal diseases entering the country.

Commenting on the introduction of these checks, the letter said: “All parties involved in moving plants, trees, flowers, bulbs and seeds between the EU and the UK are now doing so under maximum pressure, facing unmanageable increases in costs, experiencing painful border processes yet with minimal information with which to do it.

“We recognise the need for a strong biosecure system yet one that still facilitates and encourages legitimate trade.”

The letter was prompted by an incident at the government’s Sevington border control post in Ashford last week, in which several lorries transporting plants from Italy were held for more than eight days over fears they were transporting pests into Britain.

The Guardian understands that traces of the Pochazia shantungensis pest had been found in eight consignments, with these being given destruction notices. The government said that all consignments that had been held up and tested were found to have traces of the pest and these robust checks had stopped them from entering the country.

However, affected suppliers have been highly critical of the lack of communication as to why the goods were being held, with the HTA saying the lack of clear information was “completely unacceptable”.

Vincenzo Marini, the chief executive of the haulage company Marini, which sends 15 lorries a week to Britain, told the Guardian that one of his drivers was forced to stay in Kent, near Sevington, for six days after his company received no information on why their load had been held and when it would be released.

Speaking last week, he said: “The lorry has now been blocked for eight days and today I had to switch drivers – the first one was sleeping in the lorry all this time.

“How could I leave him there without any information on how long this would take? We are kept in the dark about everything.”

Marini, who has been sending good since the 1980s, added: “The UK is putting me into huge difficulties. I know other companies that have lost drivers, too, and so they have abandoned the UK due to this mess.”

Another affected Italian supplier, which preferred not be named, said it had now paused deliveries to the UK over concerns that similar holdups would take place, leaving a number of UK customers waiting for deliveries.

The company, which relies on the UK for 60% of its business, said there had been no communication about what had happened and many of the support lines were engaged when they tried to contact support agencies.

A spokesperson for the company said: “Since the checks, transport costs have gone up a lot because lorries are now having to often wait five or six hours for these checks and you have to put the prices up.

“The drivers don’t want to go because they are getting sick and tired of waiting.”

This is the latest in a line of holdups at the border, with lorries being held for more than a day earlier in the year after IT outages led to long delays of perishable goods coming into the country.

Transport en Logistiek Nederland, which represents 5,000 Dutch transport companies, told the Guardian earlier this year that some of its members could start rejecting jobs because of the new delays at the border, with the average driver being held for more than four hours.

In its letter, the HTA called for border agencies to improve communication with their service users more clearly, and called for an urgent roundtable with ministers to discuss its concerns.

A government spokesperson said: “We are committed to reducing barriers to trade and cutting red tape by striking a fair balance between business and biosecurity.

“Physical checks play an important role in helping stop harmful plant and animal diseases reaching our shores, as can be seen from detecting non-native plant pests found in these consignments”.