Conservative Cllr Duncan confirmed he was standing down following a request from council leader Cllr Carl Les in a letter seen by the Local Democracy Reporting Service (LDRS).
He wrote the group would need to harness the talent of all its members to confront challenges including the growing electoral threat from Reform but declined to comment further.
Cllr Les told LDRS it was normal practice for Executive members to be asked to stand aside so others could serve and denied Cllr Duncan had been sacked.
Cllr Duncan, who remains the Conservative representative for the council’s Norton division, has served as executive member for highways and transportation since North Yorkshire Council was created in 2023.
He was formerly the leader of Ryedale District Council and stood as the Conservative candidate for York and North Yorkshire mayor in 2024.
In a letter to Cllr Les sent to the 44-strong Conservative group, Cllr Duncan said he had been told the request to stand aside was not performance-related.
He added he had remained committed to the unity and stability of the group which had been their leading priority.
Cllr Duncan wrote: “You explained during our conversation that your purpose was to prepare for the future and to make way for new blood.
“I agree that as we approach the election, the next generation will be key to our party’s renewal.
“I continue to believe that our greatest strength lies in the wealth of talent and experience we possess across our benches.
“To meet the challenges ahead, we must be a party that harnesses the energy of all our members.
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“This will be vital as we confront the threat posed by Reform.
“To win in 2027, we must be a dynamic and forward-looking party that inspires the trust of the electorate.”
Cllr Les told LDRS he would confirm Cllr Duncan’s replacement in due course but he currently had someone in mind.
Councillor Carl Les, leader of North Yorkshire Council
He added when asked if he was concerned about the political impact of the move that it had been done to allow other members of the group to develop.
The Conservative council leader said: “I’ve had conversations with Cllr Duncan about this for the past week or so, I thought it was time for a change.
“This is something that we do on a regular, almost annual basis, a year ago I asked two people to stand down so two other councillors could take their place.
“If someone wants to develop and serve the council on the Executive then someone else has to give up their position for them.”
Cllr Duncan has overseen the project to realign the A59 at Kex Gill and the £12.6 million Harrogate Station Gateway scheme which is currently subject to a High Court challenge.
The cost of the Kex Gill project recently rose from £68.8 million to £82.5 million amid delays and rising costs.
Cllr Duncan’s letter stated that keeping the project on track and ensuring it will be ready to reopen in spring 2026 was one of his leading achievements.
He added he had also worked to protect at-risk bus routes and had drawn up proposals to extend free parking to every town in North Yorkshire.
The political make-up of North Yorkshire Council, which has 90 members, remains unchanged following the move.
The council is governed by a joint Conservative and Independent administration which holds 47 seats in Northallerton’s County Hall.
An opposition Independent group and the Liberal Democrats hold 13 seats each, while Labour holds 10 and the Greens four.
Reform have one seat and another councillor sits as a Liberal, with Easfield division’s seat currently vacant.
