A project to make the A465 a dual carriageway between Gilwern and Brynmawr is almost £51m over budget, the Welsh Government has said.
Ministers are in dispute with the contractor upgrading the road, Costain, in an effort to pay the firm only what it is contractually entitled to.
Completion of the section is now delayed until the autumn of 2019.
Costain said it continues to work "closely with the Welsh Government to deliver this complex project".
The announcement by Economy Secretary Ken Skates follows a review of the project, originally budgeted at £220m.
According to Mr Skates Costain has found the project - which involves widening the Heads of the Valleys Road through the Clydach Gorge - "far more difficult to deliver than they originally envisaged" due to the "topography" and "complex ground conditions".
Mr Skates said: "The scheme is currently projected to be delivered 23% over the approved budget.
"I am very disappointed by this but my officials are actively managing this overspend to identify ways of mitigating the project position.
"As part of this work, Welsh Government is currently in dispute with Costain on a number of matters around the allocation of risk in the contract to ensure they are only paid what they are contractually entitled to.
"I understand that this news will be concerning for those living and working in the area and I am very grateful for their continued patience and understanding while we deliver the scheme."
Welsh Government put the cash cost of the overspend at £50.83m.
The Welsh Conservatives called for clarity over how the overspend would be paid for, and when the scheme would be finished.
"These delays could cost in the region of £50m, and the Cabinet Secretary needs to clarify where the money is coming from," said Welsh Tory leader Andrew RT Davies.
"We have been raising concerns over this scheme for many months, and it's deeply disappointing to see the project slipping so far beyond schedule."
A Welsh Government spokesman confirmed the projected overspend was £50.83m, adding: "The overspend on this project will be apportioned between Costain and Welsh Government strictly in accordance with the contract terms."
A spokesman for Costain said: "We do not comment on specific commercial discussions in relation to our clients but continue to work closely with the Welsh Government to deliver this complex project."