A £27 million project to modernise facilities at Stonehenge will finally be opened tomorrow - after decades of planning.

More than a million people flock to the popular ancient monument, near Salisbury each year, with hundreds attending ceremonies to mark the solstices.

Since the end of the 19th century, Stonehenge has been ''severely compromised'' by the intrusion of roads and traffic, with ''outdated and inadequate'' visitor facilities.

Stonehenge's connection to the surrounding prehistoric landscape and nearby ancient monuments has also been cut off with roads and fences.

Work to ''restore the dignity'' of the monument as part of a £27 million project led by English Heritage began last year.

A section of the A344 road, which runs past Stonehenge, was permanently closed in June and tomorrow the visitor building - complete with a ''thrilling'' exhibition - will open.

The centre, designed by architects Denton Corker Marshall, lies a mile-and-a-half to the west of the stones.

Stonehenge is not immediately visible from the visitor centre, with tourists able to use a 10-minute shuttle bus or walk down a newly-reconnected ancient processional approach.

Further work to decommission the existing facilities, built in 1968, and returning the car park to grass will commence in January.

The transformation is the largest capital project ever undertaken by English Heritage.

Simon Thurley, chief executive of English Heritage, said: ''This is the end of an incredibly long journey.

''Stonehenge is almost certainly the most famous ancient monument in the world and up until now it hasn't really had adequate visitor facilities.

''There's been no exhibition, no opportunities for people to have a cup of tea, even.

''This is a radical change for the million people a year who come to Stonehenge.

''They can see the stones for the first time free from the clutter and rubbish that have accrued around them since the 1960s.

''They can understand them properly because we have an exhibition here that can take people back into the Neolithic period.

''We now have something that I think is worthy of one of the world's greatest archaeological sites.''

Dr Thurley added: ''In 1927 it was announced that the A344 would be closed. That was 85 years ago.

''By the 1960s there was a view that there should be a bit more for visitors and the first scheme was proposed.

''The first objectors started objecting and that has been the story for almost 55 years.''

The exhibition will be the first to explain Stonehenge to visitors, with 250 prehistoric objects - many unseen before - on loan from various collections.

One highlight is the most advanced forensic reconstruction of an early Neolithic man's face, based on a 5,500-year-old skeleton buried in a long barrow 1.5 miles from Stonehenge.

There is a 360-degree Stand in the Stones experience, using state-of-the-art laser scans, to allow visitors to experience summer and winter solstices.

Two rare 14th century manuscripts, including some of the earliest drawings of the monument, Roman coins and jewellery are also on display.

Modern facilities for the centre's visitors - 65% from overseas - include a ''bright, spacious'' cafe and shop.

In the shop, tourists can pick up souvenirs such as a £7.99 mouse-mat, £11.99 baseball cap, £300 silver bracelet or £9.99 Christmas bauble with a festive scene of Stonehenge painted on it.

The facilities are housed in a pair of single-storey ''pods'', sitting beneath an undulating canopy that mimics the rolling plains nearby.

Architect Steve Quinlan, of Denton Corker Marshall, said: ''The monument is a place for quiet contemplation but everyone coming here zips down the A303, in a 21st century way.

''We are trying to take someone from that world through to the world of the monument.

''That's a big trip.''

Mr Quinlan said the centre had been designed with a deliberate walk between it and a new car park for 500 vehicles and 30 coaches to move people gradually towards Stonehenge.

Materials for the project have all been sourced within 50 miles.

The landscape around the stones has also been improved, with the road running right past the Heel Stone closed and reseeded with grass and tall fences removed.

The Avenue, Stonehenge's ancient processional approach is now reconnected to the monument.

Culture Secretary Maria Miller said: ''Stonehenge is one of the UK's most iconic sites, undeniably worthy of its Unesco world heritage status, attracting one million tourists every year from the UK and all over the world.

''So it's only right that, after decades of indecision, we can now offer them a visitor exhibition centre and experience they deserve.

''A huge amount of work has gone into getting this right and making sure people can see the stones and their story in a whole new light.''

Stonehenge was formally recognised as a monument of national importance in 1883 and protected by the newly-introduced Ancient Monuments Act.

Visitors originally approached Stonehenge over unmapped and unfenced sheep pastures but by the mid-19th century, most began arriving on the A344 road.

At the end of the century, people were complaining of the number of wagons, horses and unruly crowds at the monument.

In 1915, it was sold at auction to local Cecil Chubb for £6,600. He presented it to the nation three years later.

A free car park was installed in 1935 and in 1968 facilities including a pedestrian underpass were opened.

English Heritage took on guardianship of the site in 1984, when it was established.

Two years later, in 1986, Stonehenge, along with Avebury Henge and the Stone Circles, was inscribed on the world heritage list.

At this time, the UK government pledged to remove the A344 by Stonehenge, which had become the main access route. It was used by 6,000 vehicles daily.

The project has been funded by Heritage Lottery Fund money, English Heritage's commercial income and philanthropic donations.

Loraine Knowles, Stonehenge director for English Heritage, said: ''This is a major milestone in a long journey to make the experience of Stonehenge worthy of its iconic world heritage status.''