"This confirmed sighting -- the first in two decades of monitoring this important and highly fragile population of killer whales -- is a really exciting development," Mark Hosford of the the Hebridean Whale and Dolphin Trust told the BBC.
The trust has been monitoring a group of orcas that ranges from the north and west coast of Scotland to Ireland's west coast and is thought to contain just nine older animals, Hosford said, and there are fears the group will eventually die off after becoming isolated from other killer whale populations.
"The west coast community is thought to be the only resident population of orca in the British Isles, and understanding their behavior and movements is crucial to the conservation of these remarkable creatures," he said.
Two males and a female were seen off Peterhead and Girdleness on Scotland's east coast Tuesday, conservationists said.
"These confirmed sightings are shattering the idea that there is an exclusively west coast community of killer whales," Peter Evans of Sea Watch said.
"They are clearly ranging further than we had thought, and presumably are seeking out prey in the region."