People with learning disabilities should be allowed to work for less than the minimum wage, says Rosa Monckton, a close friend of Princess Diana.
Miss Monckton, whose youngest daughter has Down's Syndrome, said that unless the law changed, they would be denied the human dignity that comes with work.
Writing in the Spectator, she said she wanted to see a "therapeutic exemption" for disabled workers.
Mencap said it was more important to help businesses create the right roles.
About 1.4 million people in the UK have a learning disability, of whom just 6% are in paid work, Health and Social Care Information Centre data from 2015 shows.
'In the shadows'
Miss Monckton wrote: "Services are closing, and day centres barely exist any more, so what lies ahead for people like my daughter?
"Unless the law changes, they can expect a life spent in the shadows, slumped on a sofa, eating the wrong sort of food, watching daytime television.
"This is not about the right to a minimum wage, it is about the right to have the human dignity that comes with work, and with being included."
From April, the minimum wage goes up to £7.05 an hour for 21 to 24-year-olds, and £7.50 an hour for workers over 24.
She said the ratcheting up of it was the single thing that made it most difficult to get people with learning difficulties into work, but raising the subject was considered "brutish and inhumane".
In 2011, Conservative MP Philip Davies told Parliament that disabled people should work for less than the minimum wage during a debate on the Employment Opportunities Bill.
His remarks were condemned as "outrageous and unacceptable" by a select committee chairwoman and "a preposterous suggestion" by a charity.
But Miss Monckton said that it was "so obvious" to most parents in her position that a "therapeutic exemption" from the minimum wage would have a "transformative effect".
"In practice, money isn't the real point.
"People with a learning disability may still be living with their parents.
"Very often they have no understanding of money," she added.
"They want to work so as to have a fulfilling and purposeful life."
She described how her daughter, whose godmother was Princess Diana, once asked if she could now go to New York after being given a £5 tip.Image copyright PA Image caption Rosa Monckton, photographed here in 1993, with Princess Diana, was said to be her confidante
Mencap, a charity for people with learning difficulties, disagreed that the main barrier to getting work was the minimum wage.
Educating and working with employers to create the right roles was the most important factor, said Dan Scorer, head of policy.
"With the right training and support, people with a learning disability can be valued employees whose work is of the same value as anyone else."
He added that current employment support was "completely inadequate" and the government has a lot to do if it is to improve the "appallingly low levels of employment" of people with a learning disability.
Miss Monckton set up a charity last year after she found there was nothing for her child to do after leaving college.
Team Domenica, named after her daughter, runs a course in supported employment as well as a cafe open to the public and an on-site business, both staffed by people with learning difficulties.