Work to return the final sections of 600-year-old stained glass to York Minster’s world-famous Great East Window has started today, nearly a decade after all 311 panels were removed.

The window, which is the largest expanse of medieval stained glass in the UK, has been part of a £11.5m restoration project, one of the largest of its kind in Europe, which is due to be completed next year.

In 2008, York Glaziers Trust removed all the glass from the window and between 2011 and 2017 conservators have spent around 92,400 hours meticulously conserving and restoring each piece.

In 2015, 157 stained glass panels were returned to the window as part of the five-year York Minster Revealed project, which concluded in March 2016, and over the next eight weeks the final 154 panels will be added to the Tracery and Old Testament sections.

Sarah Brown, Director of York Glaziers Trust, said: “It’s a huge milestone for the team to reach and exciting to think that, for the first time in nearly a decade, the Great East Window will again be complete.

“The window is one of the great artistic achievements of the Middle Ages, a stunning expanse of stained glass of unparalleled size and beauty in Britain. The work undertaken as part of this project will ensure this masterpiece is preserved for hundreds of years to come.”

The window was created between 1405 and 1408 by Master Glazier John Thornton, who was paid £56 by the Chapter of York. It is a work of enormous ambition, depicting the beginning and end of all things from the book of Genesis to the book of Revelation, known in the Middle Ages as the Apocalypse.

Work to restore the window originally started in 2005, when the East End and Great East Window were covered in 16 miles of scaffolding so stonemasons and conservators could assess the condition of the 14th century stonework.

Centuries of exposure to the elements had left the stone so badly weathered that the window had begun to bow. The size of the task prompted a bid to the Heritage Lottery Fund, which in turn led to York Minster Revealed – a five year, £18m project generously supported by a £9m grant from the Heritage Lottery Fund, matched by funding from the York Minster Fund and the Chapter of York.

Work to return the glass to the window started in June 2015, when ‘God the Father’ was added at the window’s apex. The message inscribed on this panel – ‘I am the beginning and the end’ – summarises the message of the window.

In addition to restoring and conserving the 600-year-old glass, the project has involved installing state-of-the-art UV resistant protective glazing. York Minster is the first building in the UK – with the largest worldwide use to date – to use the revolutionary external glazing.

The final panel is due to be returned to the window in early January and the entire project completed by May 2018.