The case for a £12 billion new cross-London rail scheme to open in the early 2030s has beeb put forward by business leaders.
The Crossrail 2 scheme would connect south-west and north-east London via a tunnel beneath the heart of the capital.
It would complement the west-to-east cross-London Crossrail project already in construction and which will be operational 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 today.
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."
Crossrail 1 was first suggested in the 1990s but was dropped on financial grounds and only given the go-ahead in 2007 by former prime minister Gordon Brown.
Bob Crow, general secretary of the RMT transport union, said: "It is vitally important that we don't waste more time delaying transport infrastructure developments that would make a massive difference for millions of people.
"If we don't crack on now, existing services will reach saturation point by the end of this decade, with stations closed due to overcrowding and trains rammed full at peak times. We are already in that position right now on many parts of the network."
He went on: "It is equally important that big business isn't allowed to call the shots on the routes and the timescales for these infrastructure developments.
"They should be built and operated in the interests of all Londoners, not just the wealthy elite."
Transport minister Stephen Hammond said: "As London grows we'll need continued investment in its transport network to accommodate this additional demand, and support continued economic growth.
"Crossrail 2 is certainly one of the options for doing this. I welcome the work done by Transport for London and the publication of London First's report and will now consider the points it raises."
Network Rail (NR) chief executive Sir David Higgins said: "If the capital's economy is to continue to thrive then we must plan now, together, for the transport infrastructure requirements of London's future.
"Our projections show that by 2031 we will need to accommodate 36 per cent more commuters into London each day. NR is already delivering the biggest capacity improvement programme since the Victorian era, but even that will not be enough on some routes.
"A regional Crossrail 2 scheme will provide the capacity we need to provide for the commuters of the future, providing extra capacity to and through central London and easing overcrowding on the already congested routes into Waterloo and Liverpool Street.
Transport for London's (TfL) planning managing director, Michele Dix, said: "We welcome London First's recognition and support for the need of Crossrail 2.
The capital needs continuous long-term investment in its transport infrastructure to keep pace with London's increasing population and to support economic growth.
"TfL has been considering the route options for Crossrail 2 in light of this as well as the impact from projects such as the Government's proposals for (high-speed rail project) HS2.
"We are currently considering two options for the route; we will be developing these options in more detail before carrying out a strategic consultation starting this spring."