Drugs minister Victoria Atkins has been accused of "hypocrisy on a grand scale" over her husband's involvement in a legal cannabis farm.
Paul Kenward - who is married to the MP for Louth and Horncastle - is managing director of British Sugar, licensed to grow non-psychoactive cannabis.
The Home Office said Ms Atkins declared the fact in parliament when she was appointed minister in 2017.
She has previously spoken out against the Class B drug.
Peter Reynolds, president of Clear, Campaign for Cannabis law reform, said: "It's not just a conflict of interests, it is hypocrisy on a grand scale."
"The reason she was appointed is Theresa May was looking for someone who was a hard-line prohibitionist," he said.
"The Tory party's policy on drugs - cannabis in particular - is directly opposed to the evidence, and Victoria Atkins is someone who supports that."
"But, what is appalling is she doesn't just want to support it for policy reasons, quite evidently, she wants to support it because her husband and family are directly benefitting from [the regulation of] it."
A Home Office statement said: "In 2016, British Sugar was granted a Home Office licence to grow a non-psychoactive variant of the cannabis plant.
"Victoria Atkins MP declared this fact in parliament when speaking in a debate as a backbencher, when she was appointed as Minister for Crime, Safeguarding and Vulnerability in November 2017, the Minister voluntarily excused herself from policy or decisions relating to cannabis, including licensing".
The company began growing a non‐psychoactive variety of cannabis at its plant in Wissington, Norfolk, last year for use in children's epilepsy medicine.
It was in the news in June after people living nearby complained of a smell of "weed" after the plants were harvested.
Speaking in parliament in July last year, she said: "I must first declare an interest, because my husband works for a company that has a license to grow non-psychoactive versions of cannabis to treat epileptic conditions in children.
"It is groundbreaking work, but I thought I should declare it, given that I will be talking about the psychoactive version of cannabis in due course - a very different substance."