The Scottish SPCA and British Divers Marine Life Rescue (BDMLR) are monitoring up to 100 pilot whales thought to be in danger of becoming stranded off the South Uist coast.
Initial reports suggest around 20 of the whales have severe head injuries.
Scottish SPCA Senior Inspector Calum Watt said, "We were alerted to the possibility of a mass stranding yesterday evening and are now co-ordinating with BDMLR.
"When pilot whales come inshore there is a very strong chance some among the group are sick or injured.
"We believe around 20 of these whales have severe head injuries but at this stage we aren't sure of the cause.
"One possibility is these injuries were sustained during a previous attempt to strand themselves.
"Pilot whales have extremely strong social bonds, which sadly means healthy whales within the pod will follow sick and injured whales onto the shore.
"At this stage we remain hopeful they will not strand themselves but our concern is the injured whales will come onshore and be followed by the rest of the pod.
"Attempting to refloat so many whales would be a huge task and if they do become stranded we'll need to decide upon the best course of action.
"The largest number of whales we've tried to refloat before was seven, which was in 1993. Unfortunately all seven returned to the shore and died."
The pod is in Loch Carnan, where 35 pilot whales threatened to strand themselves in October last year. The whales did return to sea but were later found dead on the Irish coast.
"It is incredible that a second pod, this time probably more than twice the size, has arrived in the same area," said SI Watt.
"There is no reason we know of why they would have come to the same location."
Photos courtesy of Steve Duffield, www.western-isles-wildlife.com