A French vulcanologist has warned that more nuclear tests at Mururoa Atoll in the South Pacific could cause the collapse of a flank of the extinct underwater volcano, leading to a big release of radioactivity.

The warning came as French scientists organising an appeal against President Chirac's decision to renew nuclear testing received a flood of signatures and messages of support from around the world.

Many scientists joining the protest have been angered by the way supposedly scientific reasons were given for the decision to end the moratorium.

The appeal notes that "the destruction of Hiroshima and Nagasaki and the constant development of new, 'high-tech' weapons, have done much to spoil the image of scientific progress".

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Pierre Vincent, a researcher specialised in the stability of volcanoes at the Vulcanology Research Centre at Clermont-Ferrand, says the series of eight underground tests in rapid succession could destabilise the Mururoa volcano.

"No one has taken into consideration the fact that Mururoa is a volcano," said Vincent, who studied the phenomenon of destabilisation at Mount Saint Helens.

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"Mururoa may be extinct, but the shock waves of a nuclear explosion create the conditions of an eruption, or worse. Recent study of oceanic volcanoes has shown they are constantly destabilised."

Vincent's warning has been dismissed as "daft" by Christian Lepareur, scientific director of nuclear test sites at the French atomic energy agency. "Minor collapses are possible on the coral part of the atoll but the collapse of the mountain is implausible," he said.

"I spoke of the collapse of a flank of the volcano only," responded Vincent, "To know how likely that is, we need to see the files, which are secret. The agency undoubtedly has excellent geologists, but have they examined the question of destabilisation?" Lepareur described an underground nuclear explosion at Mururoa as corresponding to a "pin prick" in the basalt.

"That's a joke," commented Vincent, "the bore may be a pin prick, but the nuclear explosion certainly isn't."

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Vincent is convinced that opposition to the resumption of testing will grow and intensify after the summer vacation.

Researchers organising the appeal against the testing say that within four days of launching an international, English-language version of the appeal last week, they were swamped with replies.

"The reaction of the international research community is very impressive. The answers are coming in piles. Whole laboratories are sending in the signatures of their entire teams by email," said Harry Bernas, one of the three physicists at Orsay University who launched the appeal.

"On one day, we got responses from 28 countries - at this time of year that's impressive. Usually, people just send in their signature, but most are also sending messages, thanking us for giving another image of France to that given by Chirac," he said.

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The appeal calls Chirac's decision "dramatically retrograde" and voices fears that the new tests "could well be aimed at adjusting some parameters in simulations that are preparing new weapons".

"If we open the door to nuclear tests in the post-Cold War world, we are opening Pandora's box," said Bernas, "Behind all this is the question - does anyone have a valid foreign policy in the world today?" Bernas and his colleagues Alain Sureau and Pierre Jaegle aim to take their campaign further, turning to the deeper political question. "It has got to go much further than interaction between scientists. We need the maximum number of people . . . not party politics or just scientists," said Bernas.

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"We are not all for absolute disarmament, but we are all aware of the fact that scientists and some politicians are working to downgrade nuclear weapons, maintain control of and begin to dismantle stockpiles. It is a long and difficult job, but that's the way forward," he concluded.