English Heritage, the Government's historic structures protection agency, reckons tourists who are drawn to medieval castles, cryptic stone circles and sites of global significance spend up to £5 billion a year in the region.

The most iconic of the welter of historic attractions are undoubtedly the Jurassic Coast and the Devon and Cornwall's mining structures, both of which have attained World Heritage status, a badge of approval also held by the Egyptian Pyramids and Australia's Great Barrier Reef.

Less heralded landmarks include reputedly the longest stone row in the world, which can be found about Burrator Reservoir on Dartmoor. Early Tudor artillery forts, prehistoric settlements on Dartmoor and early Bronze Age stone circles all contribute to what is an increasingly lucrative facet of the tourism industry.

The South West Observatory says visitor numbers to historic properties have remained broadly unchanged in the last seven years, but gross revenue levels have risen by a fifth in the same period.

Annette Cole, deputy director of South West Tourism, said it was easy to undervalue the magnetism of heritage.

She said: "Heritage is the third most popular activity for people that visit the South West.

"Around 42 per cent of people visit a heritage site when they come to the region. Six per cent of people say that heritage is the number one reason for coming down, so it really is important.

"It's undervalued because it is around us all the time and it's taken for granted. The number one reason people come here is the environment, and heritage is very much part of that."

The South West covers just 19 per cent of the land area of England, yet it has 32 per cent of the country's scheduled monuments, 26 per cent of all listed buildings and 18 per cent of the registered designed landscapes.

Including Stonehenge and the city of Bath, four of the 17 World Heritage sites in England can be found in the South West.

The mark is granted by the United Nations Educational, Scientific and Cultural Organisation (Unesco) for major world cultural and natural heritage locations considered to have outstanding universal value.

The Dorset and East Devon Jurassic Coast became a Unesco site in 2001 while, in 2006, the region's stone engine houses, wooden water wheels and countless shafts were globally recognised when the Cornwall and West Devon Mining Landscape became was the most recent site in England to be listed. The structures are seen as potent symbols of the industrial revolution.

Heritage is particularly attractive to tourists from countries still in a relative adolescence, such as the US, which have a dearth of historical landmarks.

Ms Cole added: "Overseas visitors make up just 7 per cent of visitors, but when they come they come for the heritage. Bath, Stonehenge, it's the iconic stuff they don't have."

There are an abundance of both natural and man-made features across Devon and Cornwall. Dartmouth Castle was begun in the late 14th century as a defence called "the Fortalice", intended to protect homes of Dartmouth merchants from a sea attack.

Pendennis Castle in Cornwall was built between 1540 and 1545. Pendennis and St Mawes Castle form the Cornish end of the chain of coastal castles built by Henry VIII to counter a threat from France and Spain.

The third British Tourism Week started yesterday and finishes on Sunday. Through a week of events it will increase awareness of the success and importance of the UK's fifth biggest industry. It also highlights the importance of spending money in the local economy as highlighted in the Western Morning News Think Local campaign