The Government's intention to ban experiments with hybridised animal-human embryos has come under fierce criticism from MPs, scientists and a phalanx of medical charities.

A letter delivered today to Tony Blair from 223 medical research charities warns that a ban on the creation of "chimera" embryos will hamper efforts to develop effective treatments for many incurable conditions. Scientists have proposed taking DNA from human skin cells and merging it with the cytoplasm - the non-nucleus part of the cell - of the unfertilised egg of a rabbit or cow.

They want to create stem cells from cloned embryos for studying conditions such as Parkinson's disease and motor neurone disease. However, the Human Fertilisation and Embryology Authority (HFEA) has postponed a decision on awarding licences until the Government clarifies its threat to ban such research.