Taxpayers will have to pay £1.3m in compensation to Norwich International Airport if the Northern Distributor Road is built - because the airport's current radar system would get confused by the cars on the road.

And to help recoup the cost of contributing to a new radar system which can filter out traffic, a section of the road which was originally meant to be dual carriageway could be scaled back to single carriageway.

Negotiations have been going on between Norfolk County Council and the airport over the effect the 19.5km road, which will stretch from the A47 at Postwick to the A1067 Fakenham Road, will have on the airport.

A problem is that the traffic on the £141.5m road would show up on the airport's radar system, interfering with the safe arrival and departure of planes from it.

County Hall officers say, unless a solution to that problem can be found, the airport would have to object to the road, despite having long supported the economic benefits the NDR has been touted as bringing.

Norfolk County Council's controlling Labour/Liberal Democrat cabinet will be asked next month to agree a solution - which would see the council contribute £1.3m over eight years towards a replacement system.

The council would actually provide the financing for the total cost of the radar replacement, with the airport paying back 12/20 of the total £3m cost to the council.

In a report which will come before the cabinet, David Allfrey, major projects manager, states: 'The airport operators support the NDR and acknowledge the benefits that the new road will provide.

'However, they do have a duty to protect their radar operating systems and would therefore object to the NDR, as a statutory consultee, if any impacts cannot be resolved.

'The necessary mitigation is to replace the existing radar with a new system that can filter out nearby traffic movements that the NDR will create. This filtering is something that the existing system cannot achieve.

'The airport has therefore confirmed that such a replacement will need to be dealt with in the form of a compensatory arrangement provided by the NDR project.

'The proposals take account of the early replacement of the radar system, the residual life of the existing radar and the fact that the airport would need to replace the radar at an assumed date in the future.'

The road has been on the drawing board for more than a decade. A council spokesman said the issue with the radar system had been known about for sometime and was not a surprise to the authority.

But Andrew Boswell, Green county councillor and a long-standing critic of the NDR, said: 'This is another nasty surprise for the Norfolk taxpayer and the concern is that this is just the tip of the iceberg.'

The government recently designated the road as being an infrastructure project of 'national significance', which council leaders welcomed as speeding up the planning process.

But Mr Boswell said: 'They are really rushing this through now and that is going to steamroller over the views of local people.

'At a time when the council is having to make enormous cuts and there isn't enough money anyway, you have to ask whether children and older people are going to end up suffering because so much money is being sucked into this road. This is a big issue for this administration.'

Consultation on earlier design changes has resulted in the inclusion of a roundabout on Fir Covert Road, as well as where the NDR meets the A1067 Norwich to Fakenham Road.

The extra roundabout adds around £750,000 to the cost of the project, but the council is looking to save £1.5m, which would also help cover the cost of the radar system replacement payments, by making the stretch from Fir Covert Road to the Fakenham Road single, rather than dual carriageway.

But Tim East, Lib Dem county councillor for Costessey and his group's spokesman for planning, transport, environment and waste, said: 'This suggestion would be madness. If this Fir Covert Road bit is proposed to be single carriageway, then we would be in the same situation as the Attleborough bypass on the A11, which was built to single carriageway specifications, only to be upgraded to a dual carriageway, at considerable additional cost, at a later date.'

The administration's recently published vision for the authority stressed that improving infrastructure was one of its priorities.

A planning application for the NDR is set to be submitted in November, subject to the cabinet's decision next month and any changes to the current round of consultation, which continues until September 20.

Under the fast-tracked process, the planning inspectorate will then assess the application's validity and start a public examination, expected to start in 2014 and lasting for up to six months.

A recommendation will then be made to the Secretary of State, with the council hoping work can start on the road in 2015, with the first cars on it two years later.