Trials for systems to help cool the Tube are to go ahead, it was announced today. But London Mayor Ken Livingstone warned that the �150 million investment in heat reduction schemes would "take years to deliver results".

And London Underground (LU) managing director Tim O'Toole said cooling the Tube was "a major and long-term engineering challenge". New measures for this summer include the first full-scale trial of a groundwater cooling project at Victoria Tube station in central London.

The programme will also trial a new "bore hole" cooling technology at Stockwell station in south London and new air-cooled trains, as well as more investment in tried and tested technologies. LU has also been working with the Underground's two private maintenance companies, Tube Lines and Metronet, to upgrade the existing station ventilation fan network. This programme of works has brought 50 fans, which had been allowed to fall into disrepair, back into operation - doubling the capacity of the fan network and providing cooling benefits at a number of stations across the network. Temporary portable industrial fans will be trialled at Seven Sisters and Chancery Lane stations this summer to increase air circulation and if successful will be rolled out to a number of stations in 2008.

There will also be cooling schemes at Oxford Circus, Liverpool Street, Charing Cross and Holland Park stations. In the longer term, new trains for the Circle, District, Hammersmith & City and Metropolitan lines will come with air-cooling. The first air-cooled trains are due to arrive in January 2010. Refurbished District line trains are also now benefiting from improved ventilation. The new Victoria line trains, which will enter service from 2009, will also have improved ventilation.

Mr Livingstone said: "London's Tube network is the oldest in the world and its design makes cooling the system extremely difficult. Transport for London (TfL) is pushing forward a programme to tackle the problem of heat on the Tube, after the decades of underinvestment. "This will take years, not months, to deliver results. In the meantime, there will inevitably be times when the Tube is hot and uncomfortable, but we are doing everything possible to tackle the problem in the short-term as well as funding long-term solutions."

Mr O'Toole said: "Cooling the Tube is a major and long-term engineering challenge, but we are also trying out short-term measures to help tackle the heat now." Meanwhile, TfL confirmed today there were "serious delays" to station upgrades due to be carried out by Metronet. This followed a report in London's Evening Standard that work to improve 25 Tube stations was running a total of 10,000 days late.

Metronet said today that it had been behind with some of the work but was now catching up, while TfL said it was in discussion with Metronet to see "what plans they have to deal with the backlog".