The Mayor's office says that the Northern Line extension will reduce pressure on Vauxhall station and existing Northern Line services, in addition to boosting the area's economy.
It is expected that work on the line could begin by 2015, with services in operation by 2020.
A consultation on the plans is already underway.
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The work, called Gift Horse, replaces the giant blue cockerel that previously sat on the plinth.
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