Fergus Hood, who was the SNP group’s finance spokesman, has quit the party to join the 12-strong Lib Dem group which is part of the alliance of Conservative, Lib Dem, Labour and Independents running the authority.
Cllr Hood, 57, who has represented the East Garioch ward in First Minister Alex Salmond’s Aberdeenshire East constituency since 2007, said it had been a “difficult decision” to leave the SNP group but that he hoped to be able to make a positive contribution to the administration.
The opposition SNP group – now with 27 members – remains the largest single group on the 68-seat council.
Cllr Hood said: “I was very disappointed not to be able to form an administration with SNP colleagues last May, but we had the opportunity and threw it away because of internal divisions around the leadership.
“The Liberal Democrats on the council have been a huge support in helping deliver real progress in my ward.”
He continued: “I have been the shadow finance spokesperson for some years and hope that I will now get the chance to use my skills, experience and knowledge to deliver the best possible public services to the residents of Aberdeenshire as a member of the administration.”
He added: “I spoke to Alex Salmond on Friday and told him I admired the way he has done things at a national level, but I said at a local level the party has work to do.
“We discussed it for a period of time and he gave me a number of options and left the decision to me.”
Willie Rennie, the leader of the Scottish Liberal Democrats, welcomed Cllr Hood’s decision. Mr Rennie said: “The Scottish Liberal Democrats have a strong record of delivery in Aberdeenshire and I am delighted that Cllr Hood has joined our hard- working local team.
“As part of the administration, we are working hard to create jobs and boost key local services. I know Cllr Hood will make a big contribution to our work.”
Lib Dem group leader and council deputy leader Martin Kitts-Hayes said: “I welcome Cllr Hood to the Liberal Democrat group and am sure that he will make a positive contribution to our team. I have worked with him for many years now as a colleague on the Garioch area committee and respect his track record as a committed and hard- working local councillor.
“The strength of the alliance administration has been our ability to work together without party politics getting in the way and our aim will always be to work in the best interests of all our constituents. Cllr Hood will be a strong asset.”
Hamish Vernal, the SNP group leader on the council, said Cllr Hood’s decision was “clearly personal rather than political”.
He said: “Cllr Hood, like the rest of the group, was upset about the SNP not being able to move into the administration of the council last year despite having our best-ever results.”