A "landmark" decision has been reached on a £3.5bn congestion-beating rail project for London, rail bosses said.
The project will involve major improvements to busy stations
Network Rail (NR) has received planning permission and legal powers for the long-delayed Thameslink service.
However, funding for the scheme still requires government approval before work can begin.
Thameslink will double capacity and increase from 51 to 172 the number of stations used by north-south services through central London.
NR chief executive John Armitt described the decision as a landmark.
"We are now one step closer to getting the green light for an essential congestion-beating rail project on one of the busiest parts of our network," he said.
The project will take an estimated seven years to build and passengers should begin to see the benefits within the first three years.
Thameslink route map
Anthony Smith, chief executive of rail customer watchdog Passenger Focus, said the announcement was a "much-needed and long-overdue step".
He said: "Overcrowding is already a big issue during the peak on these routes, with only 42% of commuters on these routes telling us they are satisfied with the room to sit and stand."
Michael Snyder, of City of London, said: "We have been campaigning long and hard for these Thameslink improvements and we are absolutely delighted to welcome the scheme."
There will be major improvements to London Bridge, Blackfriars and Farringdon stations, including platform extensions to cater for longer trains.
Peak-time trains will increase from eight an hour to as many as 24, using 12-carriage services rather than the present eight.
A Department for Transport spokesman said a decision on Thameslink funding would be announced by next summer at the latest.
The project is part of the First Capital Connect franchise, which is operated by FirstGroup Plc.