UK vegetable growers have warned that delays in seed imports due to post-Brexit border adjustments could have a negative impact on crops this year.

Trade associations for tomato and pepper producers in the UK claimed that new regulations governing the import of seeds from the EU were causing delivery delays of up to six weeks, which was interfering with their growing schedules and financial plans.

The Cucumber and Pepper Growers' Association (CPGA) stated that the major delays were causing “costly crop delays” for certain members, while the British Tomato Growers’ Association (BTGA) claimed the delays were endangering yields and decreasing growers’ profitability.

The problems were a result of post-Brexit rules that came into force on the 30 April and require import checks on a number of plant and animal products. These take place at border control posts (BCPs) at ports.

The rules mean that for some high-risk seeds, including tomato, pepper and oilseed rape, testing in Britain is now required, in addition to checks in the country of origin.

Dr Phil Morley, the BTGA technical officer, said UK testing could take up to 15 working days, while he had also heard of cases where seeds were being held at BCPs for long periods before being sent for testing.

He said: “Some growers report a six-week delay in seeds arriving at British propagators from when they are being ordered. This has a knock-on effect for propagators who have programmed their propagation for other crops as well, so they have to rejig their programmes.”

Morley added that this threatened food security, as delays in British production meant more imports.

Tomato and pepper growers rely on imports of seeds or young plants from EU countries such as the Netherlands.

In January, the National Farmers’ Union said the new Brexit border checks could pose an “existential threat” to growers because the checks would lead to long delays and young plants being damaged or destroyed. The Guardian has spoken to several importers of plant goods that have seen consignments damaged during physical inspections.

Commenting on the seed delays, Martin Emmett, who chairs the NFU horticulture and potatoes board, said: “We have been warning of issues for grower businesses after changes to BCPs for several years.

“The results of these concerns are now having real-life impacts on businesses ability to operate and plan ahead for next season.”

The British Society of Plant Breeders (BSPB) said the hold-ups were also preventing crucial UK trials of new oilseed rape varieties that could help produce better performing crops.

The BSPB said on Friday that seeds earmarked for the trial should have been sowed by 10 August, but had been held at border control posts.

Anthony Hopkins, the head of policy at the BSPB, said: “Oilseed rape is a crop under many pressures, and it’s UK growers and supply chains which stand to lose out if they face delays in being able to grow the latest and best varieties compared with farmers in other countries.”

The BTGA and the BSPB are now calling for the government to strike a deal with the EU that ensures mutual recognition of testing standards, so duplication can be minimised.

A government spokesperson said: “Protecting UK biosecurity remains one of our key priorities and we are working with BCPs to ensure they operate effectively and with traders to ensure checks are completed efficiently, swiftly, and without significant delays.

“We continue to work with industry on the importation of seeds, taking a pragmatic approach to supporting businesses and food security, while looking to improve the UK’s trade and investment relationship with the EU.”