Police arrested Nicholas Jake Barton at his Sydney home in June last year during a series of raids in which officers seized some 250,000 ecstasy tablets worth around 15 million Australian dollars (US$13 million; EUR 8.9 million).
The 33-year-old Briton, who is the son of James Barton and Arabella Spencer Churchill, pleaded guilty before Sydney's Local Court to knowingly taking part in the supply of a commercial quantity of a prohibited drug. The charge carries a maximum penalty of 20 years in prison.
Barton, who bears a passing resemblance to his famous great-grandfather, had originally been charged with a more serious offense of supplying a large commercial quantity of a prohibited drug, which carries a possible life sentence and a A$550,000 (US$485,000; EUR 330,967) fine.
Wearing a dark suit and lavender tie, Barton gave no reaction as he entered his plea.
He did not apply for bail, and was ordered to remain in custody until Feb. 22 when he will be sentenced alongside his co-accused, Reese Gerard Woodgate, 42, who has also pleaded guilty.
"It has been a difficult case for Mr. Barton and under exhausting circumstances," defense lawyer Charles Waterstreet told reporters outside the court. "We hope that by this plea he can get on with his life."
Barton's father James, who attended Tuesday's court session, said his son did not know the full extent of the ecstasy operation, but nevertheless "made an error which he deeply regrets."