The spokesman said the judges determined the original decision was "accurate and correct" and that Sandiford, of Gloucestershire, has 14 days to appeal to the Supreme Court, the BBC reported Monday. The clock was to begin running as soon as she was told of the verdict.
At trial in Denpasar, Sandiford, a grandmother, said she was forced to smuggle about 10.6 pounds of cocaine because one of her co-defendants threatened to kill her son. She was arrested after a flight from Thailand in May 2012 and accused of being the leader of a drug ring involving three other Britons.
When she was convicted, the prosecution recommended 15 years imprisonment but in January a panel of judges sentenced Sandiford to death by firing squad.
The British Embassy in Jakarta issued a statement saying, "We are disappointed to hear Lindsay Sandiford's appeal has been refused by the High Court in Bali. The U.K. strongly opposes the death penalty and has repeatedly made representations to the Indonesian government on this matter. We will continue to provide consular assistance to her at this difficult time."
The British human rights organization Reprieve said Sandiford was "targeted by drug traffickers who exploited her vulnerability and made threats against her children."
One of the other Britons, Julian Ponder, 43, from Brighton, was imprisoned for six years in January after being cleared of smuggling but convicted of possessing of cocaine. The two other Britons also were found innocent of trafficking, with one sentenced to four years in prison for drug possession and the other to one year in prison for failure to report a crime.