The Grotto was built around 1760 by Early of Tylney John Child as a boathouse in the landscaped gardens of Wanstead Hall.
It had a room above which famed for its interior reflective decoration of quartz, amethyst and other materials. When the park was opened to the public in 1882, it attracted the curious until it was destroyed by fire in 1884.
Child also built The Temple around the same time, now used as a visitor centre by the City of London Corporation (CoL) which manages the park. Fragments from the pre-fire Grotto are on display at The Temple.
Workers have cleared vegetation and brambles around The Grotto and are about to start on the vegetation from within The Grotto itself. This will be followed by a final phase assessing the remaining mature trees near to The Grotto and carrying out any remedial works required.
Head of forest operations at the CoL Geoff Sinclair, said: "The aim of this conservation work is to give Wanstead Park visitors better views of this important building and enhance its value as a focal point in the landscape of the park. We will be offering guided walks in the summer to give opportunities for visitors to take a closer look at The Grotto."
He thanked the Friends of Wanstead Parklands for their "valuable involvement" in planning the work.
Chair of Wanstead Park Steering Group and the chairman of the Friends of Wanstead Parklands Dwight Wood said:
"This a brilliant part of the great new push to get this 'nationally important park' off the "at risk register". All major stake holders in and surrounding the park: including the friends and associated local groups, Redbridge, Waltham Forest, Newham, English Heritage, Wanstead sports and leisure, St Marys church are all behind and pleased to be part of this positive project."
Further conservation work is planned at the park to clear brambles from Chalet Wood, considered to be prime spot to see native UK bluebells in the London/Essex area.