War on Want said pesticides on imported flowers from Colombia and Kenya were leaving workers there with skin lesions, allergies, eye problems, fainting, headaches and asthma. Some of the chemicals were alleged to cause miscarriages and malformations.

Published three days before Mother's Day, the anti-poverty charity's report, Growing Pains, said flower cutters were working shifts of 15 hours a day and were prey to sexual assaults.

The abuses were said to be occurring on farms supplying British supermarkets and shops despite voluntary codes of conduct to clean up plantations.

Flowers are likely to be the most popular Mother's Day gift with £225m lavished on seven million bunches. The UK is the joint biggest importer of flowers, with 75 per cent coming from the Netherlands, though many of those originate from Colombia, Israel and Kenya.

"But the bouquets you buy are likely to have been produced by a woman being exploited on a flower plantation in the developing world," the report said.

It continued: "Flowers imported into the UK are produced by intensive farming with pesticides under conditions that endanger the workers.

"They are then air freighted halfway round the world in cold storage and resuscitated with still more chemicals before reaching the supermarket shelf."

Although shoppers are increasingly aware of the environmental damage caused by pesticides and air miles, the report said they were "largely unaware" of the human price paid for their flowers by workers in poor countries.

In response to concerns in the 1990s, War on Want said the national cut flower industries developed voluntary codes of conduct, such as the Kenya Flower Council's standard and the Florverde scheme in Colombia. But the codes were "failing to protect workers properly", indicated by the fact that Colombian plantation owners appeared to have prior notice of inspections and coached workers on what to tell auditors.

War on Want acknowledged that the 100,000 flower jobs in Colombia and 40,000 in Kenya were crucial sources of income, but said they were poorly paid, provided little security and were hazardous.

A study of 8,000 flower workers in Bogota in 2002 found they had been exposed to 127 different pesticides, one fifth banned in the US for their toxicity.

"Aide, a flower worker, tells of an incident on a Colombian farm where workers downwind of pesticide spraying got sick and fainted. They were fired immediately." Last month 70 workers on a Florverde farm were reportedly admitted to hospital because of fumes from bonfires to keep flowers warm.

Colombian flower workers - 65 per cent of whom are women - are being paid 50p an hour. In Kenya, the wage is £23 a month. Overtime is "compulsory" and workers have to put in longer hours in the run-up to celebrations such as Mother's Day. Sexual harassment is "widespread".

War on Want called on the Government to adopt laws allowing overseas workers to seek redress in the UK for suffering caused by British companies or their suppliers.

People are urged to write to the Trade and Industry Secretary Alistair Darling to demand such a law, and to the supermarkets, and to buy Fairtrade flowers.