Asked on LBC Radio last week what London should look like in 20 years' time, Mayor of London Boris Johnson replied: "What about a bridge across the Thames that was once again habitable?"
He said: "We are certainly looking into the idea ... You could do something that would be beautiful.
"Something that would once again provide a commercial zone, a place that you could live on, a bridge that actually had residential and commercial property on it as the old London Bridge did.
"That is the kind of thing that I would like to see."
The Mayor refused to be drawn on possible locations for the bridge and would only say: "Obviously there has got to be potential for ample commercial and residential development on it, and that will to some extent dictate where it is."
This weekend the Sunday Times reported that the Mayor's policy director Anthony Browne has dusted off proposals by Antoine Grumbach dating from 1996 for a habitable bridge between Waterloo Bridge and Blackfriars Bridge.
Grumbach's Garden Bridge [see plans, PDF, 4.5MB] would include two 35-storey towers at the northern end and a tropical greenhouse at the southern end.
His proposal would span the Thames between Victoria Embankment and the IBM building on the South Bank.
The Guardian's architecture critic Jonathan Glancey described the Mayor's proposal as "potty": "An opportunity to build more costly, showy flats in central London and to serve up ever more lucrative chain shops and over-branded cafes to supposedly gormless Londoners still apparently hungry for more bland, packaged food and shiny knick-knacks."
This year the City is celebrating the 800th anniversary of the opening of the medieval London Bridge which was lined with houses and shops.