The Last Supper ready to go

To see how the painting has changed, click here.

The world-famous painting on a Milan monastery wall, which was finished in 1498, depicts Christ's last meal with his apostles.

But art experts are divided over the value of the restoration, which started in 1977.

Some say the painting has now regained "life and light" while critics believe it should have been left as it was and that the restoration is dishonest and unfaithful to the artist.

The restoration

Da Vinci's masterpiece in the refectory at Basilica of St Mary of the Graces was commissioned by Ludovico Sforza, a Milanese count, and took four years to complete. It started to deteriorate almost as soon as it was finished.

Humidity collected beneath the painting's surface, while dust and pollutants caused more damage. The brilliant colours in the painting were dulled and a series of crude restoration attempts over the last 500 years compounded the problem.

Over the last 20 years restorers led by Pinin Brambilla Barcilon have worked to repair areas where paint had flaked away and attempted to uncover fragments of the original painting which had been repainted or blackened by glue during earlier restorations.

She described the restoration as a "slow, severe conquest, which, flake after flake, day after day, millimetre after millimetre, fragment after fragment, gave back a reading of the dimensions, of the expressive and chromatic intensity that we thought was lost forever".

The restorers added colour to blank areas of the painting. They say the addition cannot be confused by the viewer with the original colour - but it is this part of the process that has caused disagreement.

'Echo of the past'

Professor James Beck of Columbia University's Art History department in New York told the Guardian newspaper: "It's taking art lovers for a ride. What you have is a modern repainting of a work that was poorly conserved. It doesn't have an echo of the past."

But Italy's Culture Minister Giovanna Melandri supports Brambilla's work and said: "With this restoration, no one can say this is no longer Leonardo."

Special dust-absorbing carpets and dust-filtering pipes have been installed at the monastery. The exhibit opens to the public on Friday after a special VIP viewing on Thursday.

Visitors must make reservations to view the painting and will be allowed inside in groups of 25 for 15 minutes.