THE extent of the work needed to save an at-risk monument commemorating the Duke of Wellington's defeat of Napoleon at the Battle of Waterloo has been revealed in a planning application.

The National Trust has submitted details of the £3.8 million project to repair Wellington Monument, which has deteriorated so badly it is now on Historic England's Heritage at Risk Register.

The charity has applied to Somerset West and Taunton Council for permission to carry out structural repairs and strengthening of masonry, as well as installing security windows and a stair handrail to improve public safety at the Grade II listed Blackdown Hills landmark.

The most critical items of work include:

•structural strengthening of the obelisk as a whole;

•structural repair of the obelisk shaft and its tip, including rebuilding bulging sections;

•replacement of smaller blade stones with larger scale masonry blocks;

•grouting and reduction of water conduits in the core;

•repointing;

•structural repairs to the staircase;

•dismantling of cladding to the curtain wall and buttresses and rebuilding with larger blocks.

The foundations to the 175-foot high Wellington Monument, the largest stone pillar in the UK and the world's largest triangular obelisk, were laid in 1817, two years after the Battle of Waterloo.

The building was completed in 1854, shortly after the Duke's death, although the top section was rebuilt in 1892 following a lightning strike.

There have been numerous repairs over the years and the monument was closed to the public from 1957 until 1962.

It has been closed again since 2005 and a fence placed around it to protect the public from the risk of stone debris.

A National Trust spokesman said: "In 1934 the Wellington Monument and 12.5 acres of land were transferred to the guardianship of the National Trust without any endowment.

"Campaigns of major repairs have been carried out by the trust during the 20th Century.

"The trust’s growing concern over the condition of the monument resulted in its closure to the public and the erection of safety fencing around the monument.

"Since 2015 the trust has commissioned in-depth scientific research to understand the problems facing the historic fabric in order to prepare the necessary consents to safeguard the built fabric and the cultural significance of this important monument.

"As a result of the above condition it was subsequently entered in to Historic England’s Heritage at Risk Register in 2016."

Experts say the first building works were "reasonably sound and well-constructed", but later phases were "of lesser quality and workmanship" and two rebuilds of the upper section "are also questionable".