A 10-mile rail tunnel underneath central London has been delayed for more than 20 years, but now mayor Ken Livingstone has launched the biggest-ever push to finally get it built.

An influential group of business and unions leaders joined him at City Hall to back the £10bn Crossrail project.

CROSSRAIL ROUTE

The line, stretching from Maidenhead in Berkshire to Shenfield in Essex, would carry 200 million people a year. Journey times would be nearly halved: on Crossrail it would take 43 minutes from Heathrow to Canary Wharf, instead of 70 minutes on the Tube.

It was first suggested in 1989, but no government has been prepared to put down the required £10bn - more than half the cost of upgrading the entire London transport network.

Trains running by 2015

"Crossrail is the transport system vital to underpin London's 21st century economic engine," Mr Livingstone said on Tuesday.

It would link the economic centres of Heathrow, the West End, the City of London and Canary Wharf in total home to 850,000 jobs.

"The City of London alone attracts 350,000 commuters every day," said Michael Snyder, chair of the square mile's policy and resource committee. "City businesses say transport is their biggest concern. Our public transport must be modern an efficient."

The Bill enabling Crossrail's construction is currently making its way through Parliament. If approved by the end of the year, tunnelling would start in 2009. The first trains would run by 2015 at the earliest.

Full-size overland trains would fit into the 20ft-wide twin tunnels - almost double the 12.5ft of the Tube. The 10-car trains would run at 62mph, the same speed as the Jubilee Line.

"We are already close to capacity on our existing rail system," Mr Livingstone said.

Crossrail would increase the Tube capacity by a fifth, he added. "It is even more important to London's long-term prosperity than our victory in winning the Olympic Games."

The new campaign for Crossrail also argues that it would link poor areas in the Thames Gateway with major job hubs.

It matters not only to London, but is a "national priority", the mayor said. Transport for London estimates that it would add £30bn to the UK's GDP over 60 years and save almost £5bn in business time.

'No longer a paper project'

Funding for the scheme would come equally from fares, taxes and the private sector, Mr Snyder said.

"Crosstail is no longer a paper project'. We are making real progress now in delivery," said Douglas Oakervee, chair of Cross London Rail Links.

The government backs the project. In his appointment letter to the new transport secretary, Douglas Alexander, the prime minister wrote he must ensure "that we have identified a clear way forward on Crossrail".

There is no Plan B, the mayor said. "I don't think there is anything else that is going to cost less than Crossrail."

The Crossrail campaign is writing to all major London and UK employers and the capital's 32 boroughs. The mayor will also address the all-parliamentary group on Crossrail on July 10. Other initiatives include a major stakeholder event and lobbying at all the main party conferences.