The clashes, which took place as Shell - the owner of the Brent Spar rig - tried to raise some of its anchor chains, immediately followed last week's North Sea conference in Denmark, at which Britain was roundly condemned by its neighbours for the operation and once again characterised as "the dirty man of Europe".
Meanwhile, the environmental group released an unpublished Shell document which it said showed that the rig could have been dismantled on land for less than a quarter of the cost officially claimed by the company.
The new battle marked the beginning of the final phase of the protesters' long struggle to prevent the dumping of the rig, which will be the first to be sunk since drilling began in the North Sea in the 1970s.
It began at 8am yesterday morning, when Shell tried to separate the rig from its anchors in order to tow it away for disposal.
Greenpeace said last night that four of its activists were tossed into the sea during the struggle - three when "a Shell speedboat rammed and drove over a Greenpeace life raft attached to one of the Brent Spar's six massive anchor chains, upending it" and one when a Shell vessel began winching a Greenpeace boat aboard, flipping it over.
An activist, Martijn Lodewijkx, accused Shell of acting in "a very aggressive manner" and added: "It was really dangerous."
In a different account, Shell put out a statement saying: "The company's concern for the safety of the Greenpeace activists was heightened this morning when a life raft with two persons on board sank after the occupants attached it to one of the Spar's anchor chains."
Shell told the Government that the cost of towing the oil rig to shore and dismantling it there, as Greenpeace wants, rather than dumping it at sea, would cost pounds 45m.
But Greenpeace yesterday released a Shell report which the organisation said contradicted this. The 200-page report entitled "Feasibility Study for the Scrapping of the Brent Spar", prepared by Smit Engineering in 1992, suggests towing the rig to a Norwegian Fjord and scrapping it at a cost of less than pounds 10m.
Shell did not submit the report to the Government because, the company says, it was "not really relevant".
It added that this was an old report which was used as "primary material for a full and thorough engineering and viability study".
It said the pounds 10m cost was accurate, but only for the actual demolition of the rig, and did not take other factors into account.
Greenpeace retorted that the detailed estimates in the study seemed to cover much of the same ground as the pounds 45m cost the company presented to the Government.
Last Friday the environment ministers of all the North Sea countries except Britain and Norway passed a resolution calling for all oil rigs to be dismantled on land.
Last night Chris Rose, Greenpeace campaigns director, said: "No sooner is the ink dry on the North Sea conference's resolutions than Shell is rushing to dump its garbage.
"John Gummer is delivering a calculated snub to his fellow environment ministers only hours after they had urged him to desist.
"Mr Gummer's message to the British public is clear: They must keep Britain tidy, but Shell can use the sea as its litter bin. This is an environmental obscenity that will anchor Britain to the bottom place in the European environmental league table."Reuse content