French firm Lafarge will carry out an investigation after a trespasser was able to scale the 400ft chimney at its Westbury Cement Works to place a Union Jack flag near the top.

A daredevil, who goes by the pseudonym Oxygen Thief, climbed a ladder attached to the defunct chimney some time over the weekend of June 19 and 20.

Pictures of the climb have been posted on a website dedicated to exploring derelict buildings, .

The daredevil wrote: “It had to be done really... Note it’s (the flag) not right at the very top, maybe four metres down – that’s because the ladder isn’t actually secure at the very top, it’s not bolted onto the wall on one side and it kind of moves a bit. Also, I was surprised to find the last few metres were constructed out of brick.”

Lafarge, which has owned Westbury Cement Works since a takeover from Blue Circle a decade ago, announced it was to decommission the plant, including the kilns and quarries, last month.

Altogether 21 employees remain at the cement depot, including two site wardens.

The French firm has now said it will conduct a full investigation, and a spokesman said: “We can confirm that a trespasser or trespassers entered our Westbury Works site over that weekend. This was without Lafarge’s permission.

“They then scaled the chimney and installed a Union Jack flag. As a company with health and safety as its number one priority we are extremely disappointed and alarmed that these people take serious risks to carry out these activities.

“As we have recently announced the site will no longer be used to manufacture cement it continues to play an important role as a working cement depot, therefore any unauthorised access presents a risk to both the individuals entering the site and our employees.

“We have taken extensive measures to secure the mothballed site and quarries. Around the chimney itself we have previously removed ground access ladders and installed additional fencing.

“We would where possible and practical accommodate authorised visits to our site. We do not however condone this sort of risk taking on our property and we will be carrying out an investigation into this incident.”

Since the factory has been mothballed, there have been full-time wardens and security contractors covering the site and quarries and more than £100,000 invested on security measures, including fencing.

Trowbridge firm Wiltshire Steeplejacks, of Court Street, was called in to remove the flag on Monday.