Tia MacGregor is today going to drop a political bombshell and become the third high-profile Liberal Democrat city councillor in nine months to quit the group.

The GP and mother-of-two, who represents Quarry & Risinghurst, said the party "was no longer working for her" and would now sit as an Independent councillor.

Dr MacGregor, who will tell her Liberal Democrat colleagues today she is resigning from the party, follows in the footsteps of former Lib Dem city councillor Paul Sargent, who now sits as an Independent.

He quit last May after losing out to Patrick Murray in a vote for a position on the decision-making executive.

A month later, Lib Dem Sajjad Malik crossed the floor to join Labour, blaming in-fighting and a lack of commitment towards building homes on the edge of the city as his main reasons.

Today, the Lib Dems' control of the city council is precarious - although the party could rely on her vote at last night's budget meeting.

John Goddard's group now runs the authority with just 16 councillors - two fewer than Labour's 18.

The Greens have eight, while the Independent Working Class Association has four.

Dr MacGregor, who broke the news exclusively to the Oxford Mail, said: "I have for some time now felt the direction the Lib Dem party was going in was different to mine.

"After long thought and careful consideration I have become convinced that the best way to serve my ward is to resign the party whip and for now to sit as an independent.

"I would like to reassure residents of Quarry & Risinghurst I will continue to represent their interests effectively."

Dr MacGregor, of Burrows Close, Headington, said there was no single reason why she was leaving the Lib Dems.

She added: "Being in party doesn't seem to be working for me. I suppose that without any party strings, I will be more able to represent people in Oxford."

Because the city council is hung, the ruling Lib Dem group is increasingly reliant on support from other groups.

Since the Lib Dems took control of the Town Hall in May last year, the group has been beset by allegations of in-fighting and personal differences.

When Mr Sargent left the Lib Dems he said: "I still share many of the principles and policies of the Liberal Democrats, but sometimes you have to go in a different direction."

Dr MacGregor, who was elected in 2004 and is due for re-election in 2008, joins a lengthy list of recent city council defectors.

Sarah Margetts left Labour to become an independent in January, 2002; Mick McAndrews switched from Lib Dem to Labour in May, 2000; and Bob Hoyle left Labour to join the Lib Dems in April the same year.