IT may be more than two years and three months late, but the longest busway in the world is at last set to open in Cambridgeshire on Sunday.

It may be a triumph for perseverance at Cambridgeshire County Council - and there may still be legal battles ahead about the 75 per cent construction cost overrun - but Liberal Democrat politicians are determined to rain on the Tory-controlled council's parade.

Nonetheless, users of the 25-km busway, most of which is built over the route of the disused railway line between St Ives and Cambridge Science Park, will have access to one of the country's first multi-operator smartcards to pay for their tickets.

The new ticket, which will enable passengers to use whosever bus turns up first - Stagecoach's air-conditioned, leather-seated wifi luxury or Whippet's Ryanair-style economy class - will complement the two operators' dedicated tickets, which will inevitably cost less.

The smartcards - which cost between £18 and £30 initially - are designed to provide 10 single journeys, the unit cost of which will depend on the distance travelled. They can be topped up on board in a similar way to pay-as-you-go mobile phones. Details, along with routes and timetables, are at www.thebusway.info.

But the county's Liberal Democrat opposition says each minute saved by using Cambridgeshire's the new busway will have cost £28million, and some current services on the A14 are five minutes quicker from Huntingdon to Cambridge than Stagecoach's 55 service on the guideway.

But the 55 also serves the residential heartlands of both Huntingdon and St Ives, so the comparison made with Whippet's 1A is not a direct one.

The party's Huntingdon parliamentary spokesman Martin Land, who stood against MP Jonathan Djanogly at last year's General Election, said that, with the bulk of the core cost of the guided busway falling on central government and developers, it was difficult to assess the scheme's value for money.

He compared the 38-minute bus journey on the X5 service from St Neots to Cambridge with the 66-minute journey from Huntingdon by way of the new guideway.

Mr Land said the council had missed an opportunity to provide a park-and-ride site in Huntingdon and to persuade the bus operators to run express services with just three or four stops to complement the town services.

"Perhaps a new operator will one day be persuaded to come along and offer such a service," he added.

Cheap-and-cheerful Whippet's director Peter Lee said his company would be offering weekend passengers a £1 flat fare per journey not just on guided buses but on the rest of its Cambridgeshire scheduled network.