A radical Government re-think on the law governing hybrid embryos will allow scientists to carry out virtually any work they like - if it is approved by regulators.

The move opens the door to experiments involving every known kind of human-animal hybrid. These could include both "cytoplasmic" embryos, which are 99.9% human, and "true hybrids" carrying both human and animal genes.

In addition "chimeras" made of a mosaic-like mix of cells from different species, and "human transgenic embryos" - human embryos modified with animal DNA - will also be allowed under licence.

Provision has also been made for the regulation of hybrid embryo research to incorporate any unforeseen developments that might arise in the future.

Ministers have moved a long way from original White Paper proposals for an outright ban on all human-animal embryos, prompting outrage from scientists.

The new measures are contained in a revised version of the Human Tissue and Embryology Bill which will be included in the Queen's Speech next month. They have been set out as part of the Government's response to a parliamentary committee's verdict on the Bill.

Another major change is that plans to combine two regulatory bodies into a single authority have been dropped. The Human Fertilisation and Embryology Authority (HFEA) and Human Tissue Authority (HTA) will now remain separate entities. They were to have been fused together to create a new Regulatory Authority for Tissue and Embryos (RATE).

The Scrutiny Committee of MPs and peers that reported on the Bill in August had recommended putting the issue of whether or not to allow animal-human embryos to a free vote in both houses of Parliament. It said if a decision were taken in principle to permit such research, it should be up to regulators to decide what experiments can be licensed.

The revised Bill does more than even the committee asked for. It effectively removes the barriers completely, permitting the creation of all four currently envisaged types of hybrid embryo, subject to a licence being granted by the relevant regulatory authority - in this case the HFEA.

Allowing scientists to work on human-animal hybrid embryos will greatly speed up progress in stem cell research. Researchers hope to use embryonic stem cells (ESCs), which can transform into virtually any kind of body tissue, to investigate the causes of human diseases and develop new therapies for currently incurable conditions such as Parkinson's and type 1 diabetes.