Former Gosport Borough Council Labour councillor Linda Batty has switched because of her backing of Tory Lynn Hook to become mayor next month.
She wanted to officially second a motion backing Cllr Hook after it was put forward by Tory Cllr Stephen Philpott.
But when Labour refused to allow her to support Cllr Hook, she quit.
After speaking to leader Mark Hook, Cllr Batty, who represents Bridgemary South, decided to join the Conservatives.
She said: ‘The Labour Party did not want me to second a friend becoming mayor.
‘That is despite Cllr Graham Burgess, a Tory councillor, seconding a Labour councillor becoming mayor in 2009.
‘I was upset by it but that is the way Labour is. Even though I have changed party, I will do the best for my ward as I have done for the last two years.’
Cllr Batty will be made deputy mayor during the mayor-making ceremony in May. This means she will then be mayor in 2017/18.
The rules were relaxed by the council to allow Cllr Batty to become deputy mayor as normally councillors need to serve for four years.
When she becomes mayor, she will be in her fourth year.
Labour leader at the council June Cully said supporting opposition to become mayor was not party policy.
‘Cllr Batty has obviously been planning this for a while because she stopped coming to party meetings,’ she said.
‘For her to become deputy mayor, she had to second Lynn Hook for mayor. That is against our policy.’
Cllr Cully added Cllr Batty should have resigned and stood for re-election next month for Bridgemary South.
‘She has slapped in the face the people who elected her and us Labour councillors,’ she said. I feel for the people who elected her and the people who put in the time to secure her a seat two years ago.’
Leader of the council Mark Hook said he was disappointed at the Labour party’s decision. He said: ‘It’s disappointing what’s happened especially on something that remains apolitical.
‘It’s a shame because all the public wants are politicians who work together.’
Cllr Hook added: ‘We are delighted that Linda has joined us. She has had a difficult time within the Labour party and it is unfortunate that they refused to allow her to second the new mayor.’
The new make-up of the council sees the Conservatives hold 22 seats, the Lib Dems with six, Labour with five, and Ukip with one councillor.