The company, whose current £20bn of borrowings are all guaranteed by the taxpayer, is already planning to make a partial break from state support during the next five-year regulated period from April 1 2009.

Iain Coucher, chief executive, is working on plans to raise £4.3bn of the rail group's anticipated £10bn of fresh loans without any guarantee – despite the credit crunch.

He has bigger ambitions for the subsequent five years. "Once we get to the end of this control period we will raise all new debt without a guarantee," he said. "It's part of our journey to become more grown up."

Mr Coucher said Network Rail could not accept the Office of Rail Regulation's final determination for the next five years until the company had a rating from the credit agencies enabling it to raise debt by itself.

"There isn't a dispute but we can't accept the determination until we have been through a credit rating process," he said. "We need investment grade. We would like to get single A but triple B would probably be OK."

The regulator has allowed Network Rail £26.7bn revenues over the five-year period, mainly from £16.5bn of government grants.

Credit rating agencies are assessing whether such income is enough to give Network Rail an investment grade rating, given its anticipated £28.5bn of spending on the network.

Mr Coucher admitted that "with the markets, it's tight". He is talking to the regulator over "what we would have to do if we failed to get the credit rating".

Network Rail is making its final preparations this weekend for the biggest timetable change in a generation, leading to 1,000 new train services a week.