Analysing government figures, Save the Children concluded more than 870,000 women looking after a child a home would rather be in work instead.
Urging people to "fundamentally question the figures", Claire Paye from Mothers at Home Matters told Premier: "Most mothers who are at home full time want to be at home full-time.
"There are some mothers who would like to work but actually they would rather care for their children themselves, or other mothers who would rather have family looking after their children but no family are available so they have to put them into children.
"It's a really nuanced situation."
Save the Children concluded that inconvenient, unreliable and unaffordable childcare were major issues for stay-at-home mums wanting to find work.
Steven McIntosh, Save the Children's director of UK poverty policy, said: "Evidence that the childcare system in England is not fit for purpose keeps mounting up.
"We know that families with pre-school children are hardest hit. The cost and complexity leave them stressed and struggling to make ends meet at the most important time in their children's lives.
"The government must urgently examine how to bring down childcare costs and ensure that families, particularly those on the lowest incomes, can get the support they need. It's time to make childcare work for families."
Claire Paye said too great an emphasis is placed on encouraging mothers to find work after having a child, adding that cultural attitudes need to change.
She said: "There's a sense in the country... that all mothers should be doing paid work [and] that what they're doing at home has no value because it's not being paid.
"The value of mothering is not being recognised."
Click here to listen to Premier's Rosie Wright speaking with Claire Paye at Mothers at Home Matters:
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