Husband ambushed wife and 19-year-old daughter in horror murder-suicide.

A father-of-three has fatally shot his teenage daughter and wife of 26 years before turning the gun on himself in a swimming pool carpark in the UK.

On Tuesday morning builders’ merchant Lance Hart, 57, reportedly stalked wife Claire, 51, and daughter Charlotte, 19, as they walked to their car in Spalding, Lincolnshire.

It is believed he knew the pair were going swimming, and hid quietly under their vehicle until they returned.

According to friends, the couple were experiencing marital troubles, and The Telegraph reported Claire had told him days prior to her murder she was planning to leave him.

Charlotte and Claire Hart. (Image: Instagram)

Heartbreakingly, Charlotte was living at the University of Northampton, but was only home with her parents for the summer holiday when she was murdered.

The Hart's five-bedroom family home had recently been sold for £240,000 ($420,000 AUD), which neighbours say was connected to the extreme strain on their marriage.

"I heard they were having some big marriage problems and that’s why they sold the house," one neighbour told The Sun.

A friend of the family believed the husband and wife relocated in hopes of reinvigorating their dying marriage, which began its decay when their adult children moved out.

"The parents were looking forward to a fresh start in a smaller new home," he said, adding that "Claire was chatting about the move just last week. She seemed upbeat and matter of fact about it".

Charlotte was a student at the University of Northampton. (Image: Instagram)

“She said, ‘We’re at that stage in life, a crossroads and were moving forward and looking forward’."

Another neighbour told the publication Lance had a history of violence, and once threatened him with a shotgun over a planning application.

"I was on my land and he walked across with a 12-bore shotgun saying, 'me and you have got a problem'.

"He was clearly trying to intimidate me so I said 'why have you brought that?' We started arguing and eventually he put it down."

Together, Lance and Claire also had two sons - Ryan, 25, and Luke, 26.

For support against family violence and suicide, please visit Lifeline here or call 13 11 14.