Michael Meacher, Labour's environment minister, said earlier on BBC Radio that a "pesticide tax is one of the things we are looking at," and could be included in its Budget on 2 July.
Stuart Shields, a spokesman from Zeneca's agrochemicals division, which represents around a third of the group's pounds 5.4bn total sales last year, echoed the view of the British Agrochemicals Association which said yesterday that such a tax would be "indiscriminate and unnecessary".
Mr Shields said that any tax ignored improved practices in crop management which had reduced the levels of agrochemicals used: "We are opposed to the imposition of a tax. If it is to reduce the amount of crop chemicals used, Mr Meacher should know that volumes are a fifth lower than they were 10 years ago."
Mr Shields said a UK tax would probably relate to Zeneca's full range of agrochemicals, which includes pesticides, herbicides and fungicides. However he said the impact on business would depend on whether it was imposed on manufacturers or crop growers.
Zeneca makes well over half of its agrochemicals in the UK, but only 3 per cent of sales are to UK growers.
Mr Shields said crop growers would be likely to accept higher prices because of the benefits of using chemicals on crops.
The news came on the same day that Zeneca announced it had successfully concluded its pounds 45m offer for Mogen, the Dutch crop biotechnology company.