The move - which would affect nurses and radiographers, as well as women workers in the nuclear industry - is just one of a series of controversial changes being pressed on ministers by the Health and Safety Executive (HSE).
Another of the proposals - in a consultative document now on Deputy Prime Minister John Prescott's desk - would allow nuclear installations such as Sellafield to increase radioactive discharges or expand operations "by the back door" without having to justify them to ministers or the public.
If accepted, the plans are due to be implemented on 1 January. They arise from a new European Union directive but opponents say that the HSE has gone far beyond the directive in weakening safeguards to workers and the public.
The change proposed for pregnant women is likely to cause particular outrage. At present all "women of reproductive capacity" working with radiation are protected by specially low exposure limits. Once they become pregnant, the limits drop even more.
The new regulations would end the special protection given to all young women. They tighten the limit for pregnant women, but only after they inform their employers in writing of their condition. Inevitably, protection for women in early pregnancy, when their babies are thought to be most vulnerable, will be weakened.
Helen Wallace, of Greenpeace, said: "Most women do not know that they are pregnant until two weeks after conception and usually want to wait for another month or so, to see whether they keep the baby, before telling anyone. Many prefer to wait until the first three months are up.
"As a result the early stages of pregnancy will not be protect-ed at all under the planned regulations, and this is the most important time for the baby."
The International Council for Radiological Protection, the ultimate international authority in the field, warned in 1990 that "irradiated foetuses seem to be susceptible to childhood leukaemias and other childhood cancers".
It added that it would be wise to expect this to happen "at even very low doses", and that the risk seemed to be "substantially greater" in the first three months of pregnancy than later.
The HSE accepts that the new regulations would lessen protection in early pregnancy but says that "current thinking suggests" that foetuses were not as vulnerable in the first three months of pregnancy as had been previously thought.
Another hugely controversial provision in the proposed regulations would overturn a five-year-old High Court ruling that all nuclear activities that produce radioactive waste would have to be "justified" by showing that their benefits to society outweighed the harm.
Environment ministers are at present using these criteria to assess two controversial applications from British Nuclear Fuels. One would increase some radioactive discharges from Sellafield. The other would start up a new plant to make nuclear fuel from plutonium, which many experts fear would increase the chances that terrorists could get hold of the raw material for nuclear bombs.
Under the new rules, ministers would not have been able to consider the applications like this, because only entirely new processes would require such justification. Both the discharges and the new plant would fall under 121 existing processes listed by the HSE as exempt.
Greenpeace said such a "backdoor approach" went far beyond the requirements of the EU directive. It added: "Proper openness and transparency could not be exercised under the regulations as proposed. Public anxiety would therefore fail to be addressed." The HSE said that the proposed change was a matter for ministers to decide.Reuse content