High Speed 2 is an increasingly rare example of an issue on which there is a consensus among the three main parties. But Peter Mandelson's recent rejection of the project as an "expensive mistake" and the 25% rise in its estimated cost to £42.6bn (even before a shovel has touched the ground) has created the space for a more nuanced debate about its costs and benefits.

So it is striking that both Vince Cable and Ed Balls are now making sceptical noises. Cable told Today this morning:

Well, the case for High Speed 2, which is still being made – I mean, the figures, as you know, are being revisited – have to meet a standard of cost/benefit analysis which the Treasury seeks, and which meet the requirements of the Green Book, as it’s called, on public investment.

And Balls told the FT:

We need to keep a close eye on value for money. I am concerned about the rising costs. As a Leeds MP I can see the benefits for the region and the north of England, but it is not a blank cheque...we have to know the benefits justify the expenditure, so therefore value for money continues to be an important test for me.

It would be wrong to assume that Balls is preparing the ground for a Labour U-turn. Ed Miliband remains personally supportive of the project and HS2 evangelist Andrew Adonis, the party's shadow infrastructure minister, who Miliband has just appointed to lead a growth review, is also determined to prevent any backsliding. But it is no longer unthinkable that one or both of Labour and the Lib Dems could go into the next election pledging to scrap the scheme. In these straitened times, £42.6bn is not to be sniffed at. As one Labour MP recently put it to me, "just think how many houses we could build with that".