Jasmine Hartin, the partner of Lord Ashcroft’s son, was taken into custody after the body of a superintendent was discovered on a dock in the central American country on Friday.
Police said Henry Jemmott, who was found dead in the town of San Pedro, had been fatally shot.
His body was found in the water with an apparent gunshot wound behind his right ear, a police commissioner was quoted as saying in local media.
Ms Hartin has been charged with manslaughter by negligence in connection with the death, her lawyer said on Monday.
“Bail has been denied,” Godfrey Smith told local media outside court. “We appeal to the Supreme Court, as is normal.”
Ms Hartin is married to Andrew Ashcroft, the son of former Conservative treasurer and deputy chairman Lord Ashcroft.
Her father-in-law holds Belizean citizenship and was once its representative at the United Nations.
Lord Ashcroft, who is also a Tory donor, sat in the House of Lords until 2015 and has retained his peerage since stepping down.
Ms Hartin’s lawyer is expected to release a statement concerning the case later on Tuesday.
Speaking about the events surrounding the discovery of Mr Jemmott’s body, police commissioner Chester Williams previously said a single gunshot was heard "and upon investigating, police found the female on a pier, and she had what appeared to be blood on her arms and on her clothing".
The gun involved belonged to police superintendent Mr Jemmott and police understand he and Ms Hartin were friends, according to local broadcaster Channel 5 news.
Additional reporting by Associated Press