Horse riding groups say they feel 'duped' after the Highways Agency changed the plans for a much-debated new underpass for the A11.

Riding and local access campaigners removed their objections to the �134m dualling of the truck road between Thetford and Barton Mills two years ago after a crossing was added to the scheme near the Elveden war memorial.

However, representatives from the British Horse Society and Camino Riders, accused the Highways Agency of 'discriminating' disabled and older riders after reducing the proposed height of the underpass from 3.4metres to 2.7metres, which would mean that riders would have to dismount and walk their horses through.

Elizabeth Barrett, of the Camino Riders rights of way group, said the Highways Agency had 'done the dirty' on them and had not asked riders about their view on the changes.

'The height they are proposing is not safe and it is a very wide road. If a big horse pulls up it will hit its head on the ceiling of the underpass and if something panics that horse, it will run off. I think it is discrimination against the disabled and the not so able,' she said.

Work on the long-awaited dualling of the A11 between Thetford and Barton Mills is set to start next year after preliminary work began in October on access roads for farm vehicles.

Brian Freemantle, of the British Horse Society, added: 'Leading a horse in a dangerous situation is the worst thing you can do. They [the Highways Agency] know they are not going to get away with it.'

A spokesman for the Highways Agency said the proposed height of the underpass had been changed because of the water table, but officials would be making further investigations following talks with the Elveden Estate and consultants.

'The safety of horse riders using the underpass on this busy dual carriageway is a priority for the Highways Agency. After consultation with our consultants and the Elveden Estate we will be carrying out further investigations into the level of the water table. Once completed we will advise our stakeholders and all other interested parties as to the outcome of these investigations,' the spokesman said.