Details for a high-speed bus route through the city centre have been revealed – but so has news of spiralling costs.

A Rapid Transit bus link from Ashton Vale to Temple Meads was initially expected to cost £35 million and be complete by 2013.

But a report going before the West of England Partnership (WEP) on Thursday shows that the scheme is now likely to cost £47.3 million – £12.3 million or 35 per cent more than planned.

And up to 10 per cent of the final cost which had been expected to come from the private sector could now have to be met by taxpayer, as the recession hits developers who were due to contribute.

The economic downturn means that funds pledged by developers in return for being given planning permission – legal deals known as "section 106 agreements" – may no longer be forthcoming as building projects are cancelled.

So the funds, which could have totalled £4.73 million, will instead need to come from the transport funds of the WEP – council taxpayers' money from Bristol, South Gloucestershire, B&NES and North Somerset.

In early December the city was given its first view of the kind of vehicles which may run along the high-speed route.

Now further details have been released about how the bendy buses will travel through the city.

The preferred route will involve an anti-clockwise loop around the city centre, after crossing the Floating Harbour over a new bridge by Prince Street.

This would take the buses, which will travel along guided busways for much of the route but can also ride on normal roads, along The Grove, up Redcliffe Way and on to Temple Circus.

Planners are still discussing the possibility of creating a new transport interchange next to the existing Temple Meads Station, on a piece of land by the Friary known as Plot 6.

From there, the most likely route is along Temple Way and Bond Street, down the Haymarket, Rupert Street and Colston Avenue, to Broad Quay and back up Prince Street.

Alternative options which could involve travelling through the Cabot Circus development or going through Broadmead at Union Street, are still being considered.

The bus would be given priority through the city centre to ensure fast, reliable services but the exact nature of these measures are still be worked out.

The report will be presented to the WEP by Bristol City Council's head of transport operations Colin Knight on Thursday.

It says: "The estimated cost of the route is £47.3 million with contingencies.

"Of this, a minimum of 10 per cent has to be funded locally.

"Originally this was envisaged to be entirely achievable through section 106 contributions, but the economic downturn may impact on the timing of some developments.

"It may be appropriate for authorities to underwrite any possible shortfall through the Joint Local Transport Plan Integrated Transport Allocation."

The councillor in charge of transport in Bristol, Mark Bradshaw, said the increased costs were the result of changes made to the scheme after public consultation was carried out towards the end of last year.

He said: "These changes include an integrated plan for the route to serve the proposed stadium for Bristol City Football Club, additional works for services to run via Redcliffe to Temple Meads, increased provision for environmental mitigation works such as vegetation planting and a new and improved interchange at Temple Meads Station."

Mr Bradshaw said work to plan the possible routes the BRT system would take through the city centre was still being done but there were a limited number of options. He said there was a possibility measures to give public transport priority over other traffic, already in place in the form of bus lanes and devices to give buses priority at traffic lights, could be expanded.

Mr Bradshaw said he was working to ensure the scheme, first mooted more than three years ago, was up and running as soon as possible.

He said: "I can understand the impatience of many people keen to get BRT in place and making a difference.

"As Executive Member, I'm grateful for the hard work that's been put into getting the proposals to this stage and for the constructive comments received during the recent consultation.

"Above all, I'll do all I can to secure this much needed public transport investment for Bristol and the city region."

The Rapid Transit services are due to run through the city every five minutes, following a course from the Long Ashton park-and-ride site, past Silbury Road, across the railway line from Portishead via a new bridge, next to Cumberland Road and alongside the Floating Harbour. Plans are also in the pipeline for an extension leading to Bristol International Airport. Work on the route, the first of several to be created across the Bristol area, is due to start in 2010 and should be complete by 2013.