Scaffolding is coming down at Canterbury Cathedral to reveal what is said to be one of the greatest conservation achievements of the 21st century.

Bosses at the city landmark launched a £2.5 million restoration project in 2009 when a hefty chunk of masonry crashed to the ground from the Great South Window.

It rang alarm bells with conservation experts, who immediately sealed off the area and began an investigation.

Closer inspection revealed cracks in the ornate stonework supporting the Gothic window, which contains some of the most precious medieval glass in the world.

Scaffolding was urgently erected to reduce the risk of the structure collapsing, and the job of finding a solution began, involving some incredibly complex work.

The decision was taken to carefully remove all 179 panes of stained glass, which depict the 22 ancestors of Christ, and put them into special storage while reconstruction of the window frame was undertaken.

Now after years of painstaking research and work, most of the stonework has been replaced to provide a secure frame for the magnificent 55ft window, which contains glass dating back to the 15th century.

Video: Work on the window restoration is almost ready to be unveiled

The project has involved an extraordinary feat of structural engineering and the hand-carving of 40 tons of limestone sourced from Poitiers in France.

The project has been overseen by Jo Deeming, surveyor of the fabric at the Cathedral, who worked closely with head of conservation Heather Newton and director of the stained glass studio Leonie Seliger.

All of the stonework was carried out by the Cathedral’s in-house team of masons, including apprentices.

Mr Deeming said: “It is a truly unique structure and we were really pushing the boundaries of conservation in what became a hugely complex task.

“We discovered one of the major causes of the cracks were the foundations beneath the window which had become waterlogged by a leaking monastic drain.

“The project also involved a lengthy planning process, but what has been achieved is an amazing feat of engineering which, I believe, will last another 200 years.”

Senior stonemason Darren McCulloch-Smith was credited with “living and breathing” the project, which included making detailed templates of the original stone carving and blocks so they could be accurately recreated.

Now the painstaking task of re-installing the precious glass panels is under way with the spectacular window due to be fully unveiled in November.

The glass is now protected from the elements by a new glazed exterior barrier which is said to work better than the one installed in the 1970s.