Phil Graham, chief executive of the government's infrastructure advisor the National Infrastructure Commission, was speaking to the National Infrastructure Planning Association's annual conference yesterday.
He said the final assessment - due to be published by the commission on 10 July - would take account of the government’s "fiscal remit". This specifies that any recommendations in the assessment should fall within a long-term cost guideline of 1.2 per cent of gross domestic product per annum up to 2050.
Graham said the assessment could not therefore be "an unaffordable wish-list".
"It cannot be something that talks about everyone’s view on what infrastructure is needed with no sense of how that can be afforded or delivered on the ground," he said. "It needs to be realistic, properly-costed and affordable."
Graham said this meant the NIA would not be able to recommend that everything was taken forward, adding: "But that’s far outweighed by the opportunity it presents and, in particular, this places the government in a position where it is far harder to step away from our recommendations on the basis of unrealism or lack of affordability.
"The government has provided us with a set of rules and a clear long-term view of a manageable level of investment.
"We’ve played by those rules and will have delivered something that shows what can be achieved with that.
"So they are very much on the back foot in terms of stepping away from that."
Graham said the NIA would analyse potential new forms of infrastructure funding, such as land value capture.
"We are looking at whether there should be strengthened or additional powers for local authorities in this area, and our final report will say something about that," he said.