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LONDON — When Jo Willoughby and Liesl Rose gathered in Anna Miles’ kitchen in February 2017, they were angry that many English girls often missed school because they could not afford sanitary products.

Three years later, their anger has inspired a movement that has forced the U.K. government to tackle what has become known as “period poverty.”

The three friends started locally, though. Miles, a teacher; Willoughby, an artist; and Rose, a holistic therapist, called three or four schools in Portsmouth, their home, around 75 miles south of London.

They asked if a constantly stocked box of menstrual products would be welcome and were told that it would be. They got a similar response when they rang more schools in the area.

And so in March 2017 they began the Red Box project — red boxes filled with pads and tampons, left with a female staff member at a school who places them in an discreet area.

“Not having access to sanitary products is undignified and humiliating. Students worry they smell,” said Gemma Abbott, a coordinator for the community-based, not-for-profit initiative in north London. “It impacts a young person's mental health and well being.”

The stigma can stay with women in later life, she added.

Jo Willoughby, Liesl Rose and Anna Miles, founders of the Red Box Project, a charity that provides sanitary products to schools in the U.K. Paul Rose

Their campaign was backed by Girlguiding U.K., an organization similar to the Girl Scouts.

One of Girlguiding U.K.'s advocates, Emily, 17, said she had personally “missed days of school because of my period.”

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She added: “I’ve been ashamed. It’s hard to ask other people for period products when there’s a stigma associated with it.”

Emily’s fellow advocate Henrietta, 15, said that while she had not been affected by period poverty herself, she knew people who had been unable to “go swimming or do after-school activities because they can’t afford anything to stop them from leaking.”

NBC News agreed not to publish their last names because they are minors.

As word about the Red Box Project spread, the group also began to campaign for the British government to take action so they would not have to.

It also inspired others to begin similar initiatives across the U.K., including Amika George, who started the campaign Free Periods from her bedroom when she was 17.

In December 2017, she organized a 2,000-strong protest outside No. 10 Downing Street in London, where British prime ministers live.

“Periods aren’t something we opt into,” George, now 21, told NBC News. “Girls were literally being held back from getting the same education as boys because they have periods and were poor.”

After committing to do so last year, Britain’s Department for Education launched a program in January to make tampons, sanitary pads and other period products freely available to all state schools and colleges in England.

The Scottish Parliament and the National Assembly for Wales — both of which legislate education in their countries — had agreed to fund similar programs.

The move in England was welcomed by Rosamund McNeil, assistant general secretary at the National Education Union, the U.K.’s largest union for teachers.

“Because period poverty is to do with girls, it has been overlooked and kept invisible,” she said. “When students are malnourished or getting to school late because of having no bus fare, that’s visible — but period poverty isn’t.”

Period poverty affected 1 in 10 British girls aged 14 to 21, according to a 2017 report by the children’s charity Plan International U.K., which also found that 49 percent of girls across the U.K. and Northern Ireland had missed an entire day of school because of being unable to afford feminine hygiene products.

Of those, 59 percent made up a lie or an alternative excuse to justify the days they were absent, the report said.

It found some schoolgirls were using makeshift products, including toilet tissue, used socks and newspapers, while others had been teased and bullied.

Period poverty is a global issue, according to Action Aid, an international charity that works with women and girls living in poverty. While some have no access to sanitary products, others are forced to repeatedly use the same items, which can cause infections, it says on its website.

While the Red Box Project was pleased about the U.K. government’s involvement, Abbott said it will now try to make sure that schools in the country embrace it.

“We are already seeing on social media that people in other countries want their own government to do the same thing," she said. "We are calling this the #PeriodRevolution.”