Extra classroom training will start in November to boost pellet spreaders' knowledge of slug biology and the environmental impact of pellets. It will be required for operators who hold the PA1 and PA2 modules, unless the PA2 was achieved between 1987 and June 1994. However, anyone who is currently qualified to apply pellets is also being recommended to undergo the new assessments.

January 2010 will see the introduction of a new a module that, combined with PA1, will qualify operators to apply slug pellets only.

City & Guilds NPTC senior manager Steve Hewitt said some users are still unaware they need PA4 to apply slug pellets legally, and assume they are qualified under a derogation for PA2 - but the derogation was withdrawn in 1994.

Some students or family members with no qualifications are still being asked to apply pellets "in the mistaken belief that there are no hazards associated with the practice", Hewitt added.

"Those who are completely unqualified and without grandfather rights (born before 31 December 1964) must achieve the PA1 as soon as possible and attend the autumn update," he said.

If successful, they will be allowed to continue to apply slug pellets until July 2011, when they need the new module.

At the same time, the National Sprayer Testing Scheme is to be extended to pelleting applicators by the autumn.

The test, which is not a legal requirement but is mandatory for members of some crop-assurance schemes, will check whether the spreading machine is set up correctly to apply pellets evenly, to the required width and at the correct rate.

Consultant Steve Higginbotham of the Metaldehyde Stewardship Group, which issues advice to farmers and growers on best practice in use of pellets, said the loss of metaldehyde was "by no means a foregone conclusion".

He added: "It is imperative that levels detected in water courses are reduced this autumn. If operators are trained and the machinery used is fit for purpose, the risks will be substantially reduced."