Temperatures on Tube trains can reach 30C

More than 3,000 proposals from around the world have been received in response to the Mayor of London's challenge which started in July.

Temperatures regularly top 30C (90F) in the deep tunnels of the network, according to a survey conducted by Liberal Democrat shadow transport minister Tom Brake last summer.

London Underground has long grappled with the problem of how to reduce heat in the narrow tunnels - some built more than a century ago - which lie up to 60 metres below ground.

So Ken Livingstone is offering �100,000 to anyone who can come up with a workable solution to cooling the underground.

COOL IDEAS Your ideas to cool the Tube

But final entries must be received by Transport for London (TfL) on Friday in order to qualify.

BBC News Online recently asked readers for their ideas on how to cool the Tube and more than 2,000 were sent in, which were passed to Ken Livingstone.

Among them were handing out ice-lollies, turning the Tube into a Venetian-style oasis and putting up posters of snowmen to make people think of winter.

The criteria for entries can be found on the Transport for London website.