A turning circle for buses in an Oxford car park could be one of the key transport measures for a £220m redevelopment of the Westgate shopping centre.

The latest proposal being discussed by developers, bus companies and councillors is the creation of a bus-turning facility in the Worcester Street car park.

The facility is crucial if the pedestrianisation of Queen Street, a key aim of both councils, is to be achieved.

However, the Liberal Democrats on both authorities are opposed to the use of the car park because it could prevent its future as a canal basin.

They are putting forward a motion to both councils insisting that a bus hub next to the shopping centre is in place before they back the latest plans outlined last year by Capital Shopping Centres.

John Goddard, leader of the city council's Liberal Democrat group, said: "We are sending a warning shot across the bows of developers, planning officers and the bus companies."

The Lib Dems hold the balance of power on the city council and could vote to reject any forthcoming planning application if they are not satisfied.

Capital Shopping Centres has been holding talks about transport for months in a bid to ensure its planning application is accepted. It is due to be submitted later this year.

On June 20, Mr Goddard is putting a motion to the city council demanding a number of guarantees, including the closure of Queen Street to buses and the provision of a bus facility adjacent to the Westgate Centre.

As well as demanding a bus hub, it says council officers should investigate the feasibility of re-instating the canal basin on the site of the Worcester Street car park.

The plan to revive a canal basin was first mooted three years ago when Oxford was competing to become Capital of Culture and the Lib Dems say council officers should now investigate the feasibility of the project.

David Robertson, the county council's executive member for transport, confirmed the car park, run by the city council and owned by Nuffield College, was being considered as a location for a bus turning circle.

The first application was abandoned following a public inquiry, triggered by opposition from Oxford Preservation Trust.

Richard Cable, director of Capital Shopping Centres, welcomed the Lib Dems' motion and added: "We are currently going through a period of extensive consultation on the proposals and are putting a huge amount of effort into ensuring that we put forward a realistic and deliverable transport solution which takes full account of all the issues, including those highlighted in their motion."