The plans would result in trains travelling between major cities at up to 220 mph.
In the west, a high speed line would run from London to Birmingham, Manchester and Liverpool. In the east a line would go to Glasgow via Edinburgh, Newcastle, Leeds, Sheffield and Nottingham.
Using Government guidelines, Atkins calculated a benefit-cost ratio of 2.0 for the project, which would see a return of £63 billion in its first 60 years of use.
Local authorities would also benefit, as the new lines would free up local services to carry more passengers.
The study also looked at two alternative high speed routes. A west-coast option would involve a high speed line from London to Birmingham only, and an east-coast option would run from the capital to Leeds. But both these provide worse benefit-cost ratios than a full network programme.
The Government has welcomed the plans but will not make any promises.
A Department for Transport spokesman said: “We will consider all options to provide the most efficient and beneficial solutions for passengers and taxpayers. In the meantime our priority is to invest Ł10 billion in increasing capacity between 2009 and 2014.”
Andy Southern, managing director of Atkins’ transport planning division, said the Government should look beyond the benefits the proposals would bring to the rail industry.
He said: “The significant costs associated with high-speed rail development need to be viewed as long-term investment in UK productivity rather than as part of the existing or future rail spending allocations.
“Given planning gestation periods, planning for lines now is unlikely to draw down significantly on existing funding allocations to 2018-19.”
Railway Forum director general Paul Martin said a high speed line would definitely happen. He said: “The Conservatives and the Liberal Democrats support the idea and rail minister Tom Harris has confirmed the door is open. The problem is this Government doesn’t feel it needs to make a decision yet.”
Atkins says a high-speed rail link could be operational by 2026 if preparatory work begins soon.