A British Virgin Islands judge ruled today that a former Rhode Island dive shop owner must serve at least 25 years of a mandatory life sentence for killing his wife on a Caribbean scuba outing in 1999, rejecting his lawyers' bid for leniency.

Justice Indra Hariprashad-Charles said the premeditated nature of the crime bound her to deny a defence request that David Swain become eligible for parole after 18 years in prison.

"It is my view that this murder was carefully planned and premeditated and calls for stiff punishment," the judge said.

The judge granted Swain about two years credit for time served. The 53-year-old is to serve his sentence at a Balsam Ghut prison on Tortola, a mountainous island about 90 miles east of Puerto Rico.

A jury unanimously convicted Swain on 27 October of murdering Shelley Tyre in what authorities portrayed as a near-perfect crime.

Tyre's drowning near an isolated shipwreck at a depth of 24 metres (80 ft) was initially ruled an accident, but authorities in the British Virgin Islands charged Swain with murder after a 2006 civil trial in Rhode Island found him responsible for her death.

The civil jury awarded Tyre's family $3.5m (£2.1); Swain filed for bankruptcy and has not paid the sum.

In the criminal trial, prosecutors argued that Swain killed Tyre to pursue a romance with another woman and get his hands on his wife's money.

Swain's lawyers plan to appeal the verdict. His daughter, Jen Swain Bloom, said evidence that would have helped her father was improperly barred from the trial.

"My family, and my father's friends and colleagues are still 100% in support of my father, and his innocence," Bloom wrote in a letter to local journalists. "No one that has spent any time with or actually knows my father thinks he should spend one more second behind bars."

No eyewitnesses or DNA evidence linked Swain to the murder. The prosecution's case rested largely on experts who testified they believed Swain wrestled his wife from behind, tore off her scuba mask and shut off her air supply while they swam near the shipwreck.

Her mask was damaged, the mouthpiece of her snorkel was missing, and her fin was found embedded in a sandbar all signs of a struggle, prosecution witnesses said. The defence called it a weak case that lacked physical evidence and was built on speculative theories and circumstantial evidence designed to roil the emotions of the jury.