The Conservatives have been accused of breaking a gentleman's agreement to rotate the role of Swindon Mayor between parties.
Before this month's local elections, Labour councillor Teresa Page was preparing to take over the role of deputy mayor which would have seen her become mayor the following year.
But on June 10 she lost her St Philip seat and the Conservatives say they intend to fill the post with one of their own.
Labour leader David Nash said:"This is the first of many promises I expect them to renege on."
"They promised us before the election that the deputy mayor would be a Labour nomination.
"The fact that our nomination lost her seat should not change what was a gentleman's agreement."
Deputy council leader Ian Dobie (Con, Haydon Wick) said the Tories had held just four of the previous 24 mayoral posts and it was time for a change.
He said the Tories' nomination for deputy mayor would be announced in time for the Annual Council on June 25.
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