A large blue rooster currently stands on the fourth plinth at Trafalgar Square, which has become a showcase for temporary artworks.
Mayor of London Boris Johnson said: "The placing of challenging artwork amidst the historic surroundings of Trafalgar Square creates a delicious juxtaposition that gets people talking and debating, underpinning London's reputation as a great world city for culture."
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Stamp duty is to be abolished for first-time buyers on properties up to £300,000, Chancellor Philip Hammond has announced in his Budget.
The money will help pay for mental health and counselling services, regeneration in the west London neighbourhood and a new community space.
The small device was hidden in a grate in the ceiling in the only customer toilet at a Starbucks coffee shop.