A second and bigger Greenpeace vessel, the Dutch-registered Altair, arrived last night in the Shetland Islands to aid the protesters aboard the Moby Dick.
They have harried Shell workers preparing the Brent Spar for its final destination 150 miles north-west of the Hebrides.
Protesters in inflatables clashed on Saturday with workers lifting the anchor chains from the structure.
Two Greenpeace supporters were reported to have been thrown into the water during the clashes.
Worsening weather yesterday led to a stand-off between both sides and the Brent Spar stayed in position, held to the seabed 110 miles north- east of Shetland by its one remaining anchor chain.
Greenpeace said: "We will do everything we possible can to win this battle. We are still determined to prevent the dumping."
Greenpeace claims sea-dumping poses an environmental hazard and could set a precedent for the disposal of other North Sea oil installations.
Shell insists the plan carries minimal environmental risk and is the best and safest option for an unusual installation which is a "one-off" in North Sea terms.
The company said that the operation to dispose of the Brent Spar rig was continuing on schedule.
Open letter to Rifkind, page 17Reuse content