The decision was taken at the authority’s annual meeting and confirms the Conservatives can form an administration, but only with backing beyond their own group. Labour councillors voted against him.
The outcome underlines the fragile balance inside the Civic Offices following the recent election, which ended outright majority control in Swindon.
In the days leading up to the vote, Conservative figures had publicly stated there would be no formal deal with Reform UK. They had also offered opposition councillors committee chair positions, describing it as an attempt to create a more collaborative council.
However, when the leadership vote was held, Reform councillors backed Mr Sumner. Labour did not.
The numbers in the chamber mean the Conservatives cannot govern alone without at least tacit support from another group. Reform’s decision to support Mr Sumner is the clearest indication yet that some form of working relationship may be emerging.
Whether that is formally described as a deal, a confidence arrangement or issue-by-issue cooperation may matter less to residents than how councillors vote in practice.
If Reform continues to support the Conservatives on key decisions such as budgets and policy votes, many will see that as a functional partnership. The situation presents both opportunity and risk for the Conservatives.
It offers Mr Sumner a route to stable leadership after a fragmented election result that reshaped the political landscape in Swindon. The council now appears to be entering a period where cross-party negotiation is unavoidable.
At the same time, Labour is likely to challenge the arrangement, arguing that voters were assured there would be no agreement with Reform, only for Reform votes to secure a Conservative leadership.
Attention will now turn to committee appointments and chair positions.
While often seen as procedural, these roles shape scrutiny, agendas and political priorities within the council. If Reform councillors are given significant positions, scrutiny over the nature of any informal agreement is likely to intensify.
If they are excluded, the Conservatives risk straining relations with the very group whose support may be required to pass future decisions.
Budgets, planning applications, regeneration schemes and transport policy could all depend on shifting alliances within the chamber. While this may encourage cooperation, it also carries the risk of instability and political deadlock.
Mr Sumner has secured the leadership. The challenge ahead will be governing without a majority, while maintaining clarity over how decisions are made and who holds responsibility.
