Sarah Gallagher - the councillor for Eastcotts - has left the Liberal Democrats and joined the ranks of the Conservatives.
She defected over what she says is the Bedford mayor's unwillingness to listen to alternative views within his Liberal Democrat group.
She said: “For some time now, I have felt increasingly distant from the mayor of Bedford and the Liberal Democrat Council group. This is a position that I feel unable to continue with and believe that the time is right to do something about it.
"I have therefore taken the very difficult decision to leave the Liberal Democrat group with immediate effect and will be joining the Conservative group, with whom my views are listened to and are increasingly aligned.
"This will not affect the residents of Eastcotts ward in any way and I will continue to represent them as their local councillor.”
Conservative group leader Cllr Graeme Coombes said: “I know this can’t have been an easy decision for her to make. I am sure she will make a very valuable contribution to our group, whilst continuing to serve the residents of Eastcotts ward as she has done for many years.
"Sarah’s disillusion with the mayor of Bedford is perfectly understandable. Many Bedford residents also feel increasingly ignored by the mayor as he pushes increasingly unpopular policies on issues such as East West Rail and new housing developments."
Christine McHugh, Bedford Liberal Democrat group leader, said: “It is true that Sarah has been distant from the group since she was demoted from being a portfolio holder.
"Whether group members are portfolio holders or not we expect them to work for the good of the whole borough as well as their local residents.
“It is a strange choice to join the local Tories who now reject the two northern routes for East West Rail, having previously supported them, so presumably the Tory group would like to see one of the three southern routes - all of which would go through her ward of Eastcotts.
“It’s also a strange choice to join the Conservatives as it is the Conservative government who are forcing housing numbers on the borough which impact villages such as Shortstown, Cotton End, Cardington and Cople.
"Yet the Conservative government ignored Bedford borough in the budget while they invest in levelling up elsewhere."