Ex-chancellor Alistair Darling has warned of a potential "nightmare" on England's existing railways if the multibillion-pound HS2 line is built.
He told the BBC the rise in projected costs from £32bn to £42.6bn would drain cash from other lines.
Labour's front bench backs the high-speed rail project but some senior figures have questioned it.
Ministers say it is vital to cope with rising passenger numbers and will create billions of pounds of benefits.
The first phase of the scheme would see 250mph trains running on a new railway line to be built between London and the West Midlands by 2026. A second phase would see the line extended further north with branches to Leeds and Manchester by 2033.
'Falling apart'Mr Darling, who was also transport secretary in the previous Labour government, was chancellor when the first stage of HS2 was approved in principle in 2010.
He told the BBC that at the time he thought the case for HS2 was "just about stateable". But he had since changed his mind because of the revised costs - which he said meant future governments would not have the money to spend on other lines.
"My experience as transport secretary is that if you do not spend money on upgrading, improving the track, improving the trains, then the thing will simply, eventually start falling apart, as it did by the mid-1990s."
He used the example of the West Coast Mainline, which he said had been ignored for 30 years by successive governments and eventually became "an absolute disaster" which had to be rebuilt at a cost of £8bn.
"That is an example of the sort of money you need to be spending. The East Coast Mainline, which was last done up seriously probably about 25 years ago, it will need money spent on... that is my concern. We're spending a lot of money on something that still isn't finalised and I also question whether or not we would get the gains claimed for it."
'Blank cheque'He also said it was wrong to assume people on trains "are not doing anything" - as wi-fi access on trains meant they were often working on laptops, probably getting more done than in the office. "I just think that the whole basis on which this calculation is made is questionable."
Connections between cities outside the South East were "not good", he said. "That's where I think any government in the future is going to have to spend money, and that's why I question whether or not you want to put all your eggs in this one basket?"
"My fear is, if you build this visionary project that you will have a nightmare on the rest of the network because you don't have the money to spend on it."
Some Conservative MPs whose constituencies would be affected have also raised concerns and several Labour figures have questioned its benefits - including another former cabinet minister, Lord Mandelson, who has warned it could be "an expensive mistake".
Shadow Chancellor Ed Balls has also said he is "concerned about the rising costs" and said the project must not be given "a blank cheque".
Mr Darling said he was speaking as a backbencher and recognised he was in a minority of MPs opposing the project.
Reports in the Financial Times this week suggested that Treasury officials were preparing for an overspend of £31bn on the project, including account inflation and VAT over 20 years - but Transport Secretary Patrick McLoughlin said the budget of £42.6bn had not changed and included contingency money.
A spokesman for the Department for Transport said on Friday: "HS2 is absolutely vital for this country, providing a huge economic boost which will generate a return on investment that will continue paying back for generations to come.
"Without it the key rail routes connecting London, the Midlands and the North will be overwhelmed. HS2 will provide the capacity needed in a way that will generate hundreds of thousands of jobs and billions of pounds worth of economic benefits.
"The government is committed to managing the cost within the budget we have set for the project and to securing maximum value for money for the taxpayer, while also ensuring that preparations are properly made for the most significant infrastructure investment the UK has seen in modern times."
Source: BBC News - Business http://www.bbc.co.uk/news/uk-politics-23808993#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa
