The Crossrail 2 scheme would connect south-west and north-east London via a tunnel beneath the heart of the capital.
The case for a £12 billion new cross-London rail scheme to open in the early 2030s has been put forward by business leaders.
It would complement the west-to-east cross-London Crossrail project already in construction and which will be operational in around 2018.
Put forward by London First, the Crossrail 2 scheme envisages a new rail line taking in such areas as Wimbledon, Kingston, Twickenham, Hackney, Islington, Tottenham, Cheshunt and Hertford East. It would also link with major London interchanges including Euston, Victoria and Clapham Junction, and reduce pressure on congested Tube lines. In some cases, journey times would be more than halved.
The opening date of the early 2030s, with construction beginning in the 2020s, would be dependent on the necessary planning and consultation beginning now, said London First - the business organisation putting forward the plan.
London First, whose task force has been chaired by former transport secretary Lord Adonis, said that without Crossrail 2, at least £6 billion would need to be spent on incremental improvements to existing Tube and rail infrastructure, "offering a fraction of the benefits while still leaving London congested".
London First chief executive Jo Valentine said: "The UK faces a stark choice: go on investing in London's transport to keep pace with population and jobs growth - or stifle London's future success with bottlenecks. And we have to make the decision now. We cannot afford the decades of indecision that delayed getting started on Crossrail 1."
Lord Adonis said: "Today's report sets out a compelling plan with strong business support."
He added that Crossrail 2 was "essential to keep London moving as its population rises by another 1.5 million over the next 20 years and the number of rail journeys into London termini increase dramatically."
London mayor Boris Johnson said: "The case for the construction of Crossrail 2 is incontestable and is made forcibly in this report. Over the next 20 years London's population is forecast to expand to levels that will clog the Tube and rail arteries of our great city if we do not provide more capacity. There is no time to lose and my team will work closely with London First and others on developing plans for this vital railway."