Mr Prescott, who once visited 10 Downing Street in a frogman's suit to protest against radioactive waste dumping, will lead the British team at a conference in Portugal which aims to start a once-and-for-all clean- up of the seas in the north-east Atlantic.
At the top of the agenda is a strict crackdown on the discharge of man- made radioactive substances, such as those produced by Sellafield's giant Thorp plant for reprocessing spent nuclear power station fuel, and a proposal which proclaims that they should be "close to zero".
Fourteen of the sixteen member states of the Oslo-Paris Commission (OSPAR), which regulates the north-east Atlantic marine environment, support the resolution. In the pre-conference negotiations, however, Britain, backed only by France, has been vigorously opposing it and sponsoring an alternative and much vaguer clause.
Furthermore, Britain is seeking to water down the proposed OSPAR timetable for a complete halt to nuclear discharges, allowing some discharges to continue.
Sarah Burton, the campaigns director for Greenpeace, said: "It is criminal that Britain is attempting to sabotage this opportunity to get rid of nuclear pollution in the seas.
"I can't believe that John Prescott, who spent many years campaigning - in the end successfully - to stop nuclear dumping at sea, should now be in the position to stop the last bit of dumping through these discharge pipes - and fail to do it."
However, Mr Prescott's deputy, the environment minister Michael Meacher, stressed: "This is a ministerial conference and we still have to decide the ministerial line that we take. To say this is the ministerial position would be premature.
"We want to reduce radioactive discharges and my wish is that they should be as low as feasible. But we need to be satisfied before making any commitment that we are capable of delivering it."
England and France, he added, were the major nuclear nations in OSPAR and in a different position from the other member states.
The treaty which Mr Prescott is due to sign at Sintra near Lisbon on 23 July will be legally binding on the signatories.
At the heart of the matter is that Britain and France, the only countries with big nuclear reprocessing plants, fear their operations might be severely restricted or even halted by a drastic cutback. "Zero" emissions would certainly close the pounds 9bn Thorp plant, which has nearly 7,000 tonnes of mainly Japanese and German spent fuel awaiting reprocessing, and France's equivalent at Cap La Hague near Cherbourg - although "close to zero" remains to be defined.
British Nuclear Fuels, operators of Sellafield, refused to comment on whether "close to zero" emissions would force the closure of Thorp. "What you are postulating is speculation," a spokesman said.
An emissions ban would have a similar effect on the the smaller and discredited Dounreay plant in Scotland, which the Government announced two weeks ago is to be shut down.