It would be "foolish and misleading" to suggest that London's Crossrail project could be done "on the cheap", the Transport Secretary has said.
Alistair Darling warned MPs that the cost of the proposed rail link, which would connect east and west London, was likely to be "pretty substantial" - somewhere in excess of £10bn.
At question time, Mr Darling said: "The cost will depend to a large extent on the nature of Crossrail itself ... I am very cautious about costs, my experience over the last 12 months is that costs in relation to railways in particular usually turn out to be rather more than people first anticipated.
"So far as Crossrail is concerned, I think it is a very important project for the future development of London but I think it is important that we get it right."
Shadow transport secretary Tim Collins called for a final decision on Crossrail before the House rose for the summer recess in July.
Labour's Oona King, MP for Bethnal Green and Bow in east London, had asked how much the project would cost and how it would be funded.
Mr Darling said it was a matter for discussion within government and with the private sector.
Liberal Democrat Edward Davey MP Kingston and Surbiton, south London, said that ongoing government evaluations of proposals from the Strategic Rail Authority and Transport for London for Crossrail simply amounted to "more delay and more indecision".
He called on ministers to spend the time looking at other options for the future.