Listed buildings in Bradford are under threat because of a lack of workers with the skills to restore them, according to research.

The study by the National Heritage Training Group (NHTG) showed that demand for maintenance work has rocketed in the past three years with the market estimated to be worth £4.7 billion.

But of the half a million professionals working in the UK only 507 are building conservation-accredited.

Yorkshire could be heavily affected as the region has more than 31,000 listed buildings and contains heritage sites such as Fountains Abbey and model village Saltaire.

The number of craftspeople in the sector is about 109,000 but with only 36 per cent of contractors working on pre-1919 buildings it is estimated that only 33,000 undertake work with traditional materials.

Every year English Heritage produces a register of at-risk listed buildings, which last year featured 11 Bradford sites including Leaventhorpe Hall in Thornton Road which is in poor condition with broken windows.

The roof was said to be sagging and the east wall failing and according to English Heritage the site had substantially deteriorated.

The Old Cotton Mill near Low Mills at Keighley was also vacant and in poor condition and Kirklees Priory Gatehouse in Brighouse was said to require urgent work due to timber decay.

The NHTG report identified about 16,000 mostly new entrants to the trade as requiring some basic skills training, suggesting that more than two-thirds of the work is being carried out by those without the right skills or materials.

Skilled carpenters, roofers, stone-masons, thatchers and dry stone wallers are all in demand.

Peter Lobban, chief executive of training body ConstructionSkills, said: "We've taken some giant steps to ensure that more people are taking up these traditional building crafts that are so important to preserving the country's heritage buildings.

"But there is more work to do. Many of the people undertaking repair and maintenance work on pre-1919 buildings need up-skilling to guarantee that tasks are completed to the highest possible standard and the iconic and more humble buildings are not spoilt."

Maddy Jago, English Heritage regional director for planning and development said: "These skills issues affect not just listed buildings, but the whole swathe of undesignated and locally important heritage and conservation areas that form an integral part of the historic environment."

The National Heritage Training Group and its partners will invest £1 million to help reduce the skills gap, to be spent on raising awareness, encouraging people to take up a Heritage Apprenticeship programme, supporting regional action groups, expanding training academies and setting up a mentoring programme.

Bradford councillor Anne Hawkesworth, executive member for environment, said: "There is a serious concern about construction skills and the level that is required. It's not just about building a wall there is a high level of ability required.

"People are simply not taking up these skills required for heritage and there are a lot of areas that need work."

Anthony Mann, property developer and chairman of the Bradford Civic Society, said: "Many Bradford listed buildings have gone to rack and ruin which is awful. Virtually every old building in Bradford city centre is listed.

"Rawson Place is falling to bits and all of its buildings are listed."