JULIA ROBERTS is to star in a film biography of Joan Root, a British environmentalist who was shot dead in her Kenyan home two years ago after she fought to save the lake she lived by from being poisoned.

Friends of Roberts said she was so shocked by Root’s death that she had decided to make A Flowering Evil, her first starring role in five years, to focus attention on the still-unsolved murder.

Police suspect Root, 69, may have been killed by gunmen hired by flower farmers she had accused of polluting nearby Lake Naivasha with illegal pesticides. A trial of two men accused of spraying her with bullets from AK-47 assault rifles collapsed last year. Police admit they have no further leads. Root, the daughter of British coffee growers, learnt to live with danger during the 1960s when she made pioneering wildlife films with her husband Alan. She snorkelled with hippopotamuses and charmed snakes out of thorn trees for the camera.

The couple introduced Dian Fossey, the Californian zoologist, to gorillas, escorted Jacqueline Kennedy Onassis over Kenya in a hot air balloon and travelled through much of Africa in an amphibious car.

The Roots divorced in 1981 but Joan continued to maintain her 80-acre plantation on the shores of Lake Naivasha, 50 miles from Nairobi, where antelope wandered through living rooms and hyenas had been known to snooze on sofas.

In recent years Root had been threatened by a new wave of black nationalists who disliked her passion for conservation. Nairobi politicians denounced her “interfering ways” and local thugs threw bricks through her windows. However, she continued to stand up against businesses she accused of blighting the environment.

In January 2006 killers broke into her bedroom, after bypassing alarms and reinforced steel doors, and shot her. Nothing was stolen.

John Sutton, her security consultant in neighbouring Tanzania, received a call from her that night. “She was saying, ‘John, help,’ and then her voice faded away,” he recalled. Hit twice in the leg and once in the hip, she died from loss of blood before help could arrive.

Roberts, a committed environmentalist, won an Oscar in 2001 for playing another environmental campaigner, Erin Brockovich, the legal clerk who won record damages against a Californian company accused of polluting a city’s water supply.

She first read about Root in a magazine article 18 months ago and returned to the prospect of making a serious film after becoming more interested in green issues following Al Gore’s An Inconvenient Truth.

Roberts is also developing a series of green films through her production company Red Om - her married name spelt backwards. She has asked her sister Lisa Roberts Gillan, 43, to find her “seriously entertaining” scripts that would allow her to act with their niece Emma Roberts, 17.

Roberts lives with her family in a small bungalow in Venice, West Los Angeles, where neighbours have noted that, when leaving the house, the actress whose films have earned £1 billion at the box office calls: “Are all the lights out?”

Explore Film

• Film Reviews

• Trailers

• DVD

• London Film Festival

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010