The Wanstead Grotto, one of the last surviving remnants of the 18th century Wanstead House, has been saved thanks to a £10,000 grant from a conservation charity.
The Grade-II listed grotto in Wanstead Park was damaged by fire in 1884 and while it has had patchy repairs since then it has been badly declining and was placed on the Heritage at Risk register.
Charity Heritage of London Trust (HOLT) has come to the rescue and has given the grant towards stabilising and restoring the historic landing stage.
Boats on the lake in front of the grotto once pulled up for guests to disembark and enjoy the beautiful entertainment rooms inside.
Restoration work will begin in the spring.
The City of London Corporation has approved a Conservation Management Plan for it - to identify the most appropriate approach to securing the structure's long term future - and further restoration will be planned.
HOLT director Dr Nicola Stacey said: "This is a spectacular and important building and we are delighted to help move its restoration forward after so many years of discussion over its future.
"The landing stage is at particular risk of further collapse so this work is urgent.
"We hope this will be a catalyst for the rest of the works to follow soon."
Richard Arnopp, from Friends of Wanstead Parklands, added: "We are thrilled to have this support from the Heritage of London Trust and also that the City of London has adopted a conservation management plan to secure the grotto's future."
Wanstead House, and an elaborate lake system fed by the River Roding was built between 1715 and 1722 by Sir Richard Child, 1st Earl Tylney and designed by Colen Campbell.
His son John, 2nd Earl Tylney added the grotto between 1760 and 1764 at a cost of £2,000.
The grotto had limestone, shells, fossils, mica chips and crystalline minerals which sparkled by candlelight and included a boat channel and dock for gondolas to pull up.
In 1884, a fire broke out in the grotto and because the lake in the front was empty while it was being cleaned, there was no water to extinguish it.
It was badly damaged with only the façade and outer walls remaining.
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