Greenpeace said the Shard protesters were "artists and activists", adding: "If the six women reach the top - 310m above the pavement (1,017ft) - they will attempt to hang a huge work of art that captures the beauty of the Arctic."
One climber, Victoria Henry, 32, a Canadian living in Hackney, London, said this morning:
We'll try to hang a huge art installation 310m up that will make Shell think twice before sending their rigs into the Arctic.
It's going to be really hard work, it's going to be nerve-shredding for all of us and we may not succeed, but we're going to do everything we can to pull it off.
Millions of people have called on Shell to get out of the Arctic but they're still trying to drill there anyway.
If we reach the top we'll be able to see all three of Shell's London offices below us, meaning they'll be able to see us.
Maybe then they'll stop ignoring the movement ranged against them.
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