Shell is suing Greenpeace for $2.1 million in damages after the environmental group's activists boarded the company's oil production vessel in transit at sea this year, according to Greenpeace and a document seen by Reuters.

The British oil and gas major filed the claim in London's High Court. Greenpeace activists boarded the vessel in January near the Canary Islands off the Atlantic coast of northern Africa to protest oil drilling and traveled on it as far as Norway.

In an email to Reuters, Shell confirmed legal proceedings were taking place when asked whether it was suing Greenpeace over the incident but declined to comment on the claim amounts.

Boarding a moving vessel at sea was "unlawful and extremely dangerous," a Shell spokesperson said.

"The right to protest is fundamental and we respect it absolutely. But it must be done safely and lawfully," the spokesperson said.

The vessel was destined for the Penguins oil and gas field in the North Sea, which is not yet in production.

Four Greenpeace activists used ropes to hoist themselves onto the vessel from inflatable boats that chased the ship at high speed.

Protests at sea against oil, gas or mining infrastructure have long been part of Greenpeace's operations.

The damages Shell is seeking include costs related to shipping delays and expenses for extra security, as well as legal costs, according to a document seen by Reuters.

"The claim is one of the biggest legal threats against the Greenpeace network’s ability to campaign in the organization’s more than 50-year history," Greenpeace said in a statement.

The group said Shell offered to reduce its damage claim to $1.4 million if Greenpeace's activists agreed not to protest again at any of Shell's oil and gas infrastructure at sea or in port.

Greenpeace said it would only do so if Shell complied with a 2021 Dutch court order to cut its emissions by 45% by 2030, which Shell has appealed.

A claim for additional damages of around $6.5 million by one of Shell's contractors, Fluor, is unresolved, according to the document seen by Reuters. Fluor did not immediately respond to a request for comment.

Shell and Greenpeace have held negotiations since the case was filed, but talks ended in early November, Greenpeace said, adding it was now waiting for Shell to file further documents in court.

Greenpeace said it will then consider its next steps, including ways to stop the case from proceeding.