High Speed 2 (HS2) bosses have insisted the London to Birmingham leg of the scheme remains within budget after route amendments were shown to cost almost £580M.
Campaigners latched on to figures released by the government showing the funding needed for a batch of changes sought to the hybrid bill for phase one of the rapid rail link.
But project promoter HS2 Ltd said savings would be achieved by the proposed alterations and that in any case an allowance had been made for such amendments.
The costs come from land purchases as well as construction work for tunnels, bridges and other infrastructure as the government seeks to fine tune the route in the face of local opposition.
An HS2 Ltd spokesperson said: “The figures in the estimate of expense are not extra costs and are not being added to the budget.
“They set out the total costs associated with the additional powers that are being sought. It does not include savings linked with those changes and so is only a partial picture of the change in costs.
“The first phase of this essential new rail link between London and the West Midlands remains within its original budget.”
The new costs relate to more than 120 design changes put to MPs last month. These include allowing the railway to pass under the A38, the West Coast Main Line and the South Staffordshire line near Lichfield rather than running over them on viaducts.
Joe Rukin, campaign manager at lobby group Stop HS2, said: “With the swish of a pen, £600M has been added to the bill for HS2, with very few people seeing any benefits from the changes, and for many the scheme has got worse.”
A total of £21.4bn has been set aside to build the first leg of HS2, which includes more than £4bn of contingency.
The target cost of phase one is £17.2bn, which includes a 10% risk allowance for design changes.