Southern Water has warned tanker companies to stop delivering its water to the Wiltshire estate of an American billionaire.

Some of it was recently used, entirely legally, to help fill a lake, despite a hosepipe ban being in place locally for domestic users.

Multiple water tankers have been filmed both day and night by local residents filling up from standpipes in part of neighbouring Hampshire where a drought order is in place.

The BBC has been told that those tankers went to Conholt Park, a 2,500 acre estate owned by Stephen Schwarzman – who is one of the world’s richest men and a financial backer of US President Donald Trump.

A spokesperson for Mr Schwarzman confirmed that a small proportion of the water transported had very recently been used to help fill a new lake but said the water has been “sourced through licenced providers responsible for the lawful and proper extraction and delivery”. They said that following the Southern Water request the supply of water to the lake had been adjusted.

Despite the ongoing drought, the tankers are legally allowed to take the water in Hampshire under licence because construction work is not domestic and therefore not covered by current drought restrictions.

However, Tim McMahon, Southern Water’s managing director, said he was “appalled by this use of water” and that the company had imposed on immediate ban on tankers extracting from the standpipes.

Southern Water said it did not know exactly how much water had been taken but that there had been a spike in the last week with “significant” amounts taken but with other users also making use of the standpipes it was impossible to say who had taken more.

The water firm, which serves more then two million customers, said it was first alerted by residents in Andover who had spotted the tankers coming and going from the standpipes.

Among them was Lawrence Leask, an air conditioning inspector who told the BBC that he has been waking up at 03:00 BST to follow the tankers from the standpipes in Andover to the estate eight miles away, just over the border in Wiltshire, which is not subject to a drought order at present.

“We think there have been over 30 tankers a day, seven days a week,” he said.

“That’s a lot of water. I worked out that 30 tankers means a million litres a day, something like that.”

He said it had been going on for the past few months, prompting him to co-ordinate a network of neighbours who took it in turns to film and follow the tankers.

One of those, Trevor Marshall, said: “We think they might be using the water to fill the lake. He makes notes of the tankers passing his kitchen window and sends them to Lawrence.

“At the same time we’re on a hosepipe ban – it’s incredibly outrageous.”

Mr Schwarzman is one of the world’s richest men, with an estimated worth of more than £30bn, having founded Blackstone, one of the biggest investment funds in the world.

He has been renovating the 2,500 acre, 17th century shooting estate since he bought it in 2022 for an estimated £80m.

That involves an extensive redevelopment and constructing a new lake.

A statement from Blackstone said: “The suggestion that the new owners of Conholt Park violated water regulations is false and misleading.

“They have taken extraordinary care to ensure the restoration of the property complies with all local laws and regulations. Most recently, as construction winds down (expected to be completed very soon) a proportion of transported water has been used in connection with irrigation and the lake.

“The water has been sourced through licenced providers responsible for the lawful and proper extraction and delivery. Water has been sourced from multiple locations, largely outside the region. Any suggestion that the owners violated local water regulations would be false and misleading.”

Southern Water said it would be “having robust conversations with those using this water and the companies working on their behalf”.

Managing director Tim McMahon said: “While this disappointing occurrence is highly unusual and rare, I would like to reassure customers that I am leading a thorough review into how this happened, and how we can tighten up both our internal monitoring processes and any legal loopholes so that this cannot happen again.”