During March, Greenpeace activists peppered underground trains with spoof versions of the "poems on the underground" posters. Poems were adapted to criticise the war on Iraq and attack oil company Esso for fuelling global warming.
The posters looked exactly like the real underground poems and made no reference to Greenpeace. The charity admitted responsibility on its web site on 31 March.
"We do not know which individuals actually put the posters up," said a London Underground spokesperson. "We have no plans to prosecute."
A total of 2,000 posters based on five famous poems were pasted onto mainly empty advertising spaces between 5 and 31 March.
"It was done in the middle of the day," said Emily Armistead, a climate campaigner at Greenpeace, who designed the posters. "We did moot the idea of going in at 5am on the first train, but it wasn't necessary. People don't question things they see on the tube."
London Underground was unaware of the posters until members of the public rang to complain three weeks into the campaign and national newspapers started to raise questions. London Underground eventually contacted Greenpeace on 2 April and the environmental group agreed to stop putting the posters up.
Armistead believes that removal of the posters may have been delayed because some London Underground staff liked them.
The campaign has provoked both positive and negative interest. "We have had quite a few phone calls, mostly complaints," said the spokesperson.
However, Armistead said dozens wrote to congratulate Greenpeace even before it admitted involvement.