Ella Mann is just 19 but she has already made her mark on the world. In September, feeling increasingly frightened by the quickening pace of climate change, she started writing a letter to one of Britain’s leading theatre companies.

It was not a decision Mann took lightly, or one that she rushed. She spent days writing the letter which would pile pressure on the Royal Shakespeare Company to drop its longstanding relationship with oil giant BP.

“I thought it was ridiculous, because Shakespeare is a key part of our identity and this identity is being stained by its association with companies that are destroying our planet,” said Mann, the daughter of a psychologist whose love of the natural world began from a young age.

Her decision to take a stand worked: just days after her letter was published, the RSC announced its “difficult” decision to end its sponsorship deal with BP at the end of this year, saying it was clear that the arrangement was “putting a barrier” between young people and the theatre.

Mann, a self-effacing gap-year student from Oxford who will study ecology and conservation biology at Leeds University next year, is one of a new generation of young activists and campaigners responding to the climate crisis in whatever way they can. She organised Oxford’s first two school strikes as the university city sought to follow in the footsteps of Greta Thunberg, the Swedish teenager who has become a global hero for the environmentally conscious.

Over the past eight years our sponsorship has enabled 80,000 young people to see RSC performances at reduced rates BP

But Mann also felt compelled to tackle the relationships between fossil fuel companies and arts organisations that, she believes, are deeply hypocritical and have – until recently – been largely ignored.

Beginning with the polite “Dear Royal Shakespeare Company”, her letter, which was signed by schoolchildren and youngsters from across the country, made a passionate plea to the theatre company to abandon its association with BP.

“We are the audiences of the future and we will not support theatre that accepts sponsorship from a company that is continuing to extract fossil fuels while our earth burns,” wrote Mann, who is spending her gap year working on the social media strategy of environmental organisations.“We wish that the RSC would act on the ideas that they present in Matilda [a recent production at the theatre] and not give in to the powerful oppressor. As said by Matilda herself: ‘If it’s not right, you’ve got to put it right!’”

Ending with the ultimatum of a boycott she signed it off simply with “Kind regards, The youth.”

BP had been subsidising the theatre company’s £5 ticket scheme for 16- to 25-year-olds. In a statement, the company said it was “disappointed and dismayed” its partnership had been brought to a “premature” end. “Over the past eight years our sponsorship has enabled 80,000 young people to see RSC performances at reduced rates,” it continued.

The company said it shared “many of the concerns that apparently contributed to the decision” and was committed to making energy “cleaner and better”.

“The increasing polarisation of debate, and attempts to exclude companies committed to making real progress, is exactly what is not needed,” it continued.

In June the Guardian revealed that Mark Rylance, who had been an associate artist with the RSC for 30 years, was resigning from his position, arguing that BP’s sponsorship deal allowed the company to “obscure the destructive reality of its activities”, which he said threatened the future of the planet.

Mann said she had been overwhelmed by the support and welcomed the RSC’s decision, but added there was a long road ahead.

“It was amazing and people responded a lot quicker than I expected. This has been a massive win and I am really impressed with the RSC for taking a stand, but the fight is not over and we will continue to try and push BP out of other organisations,” she added.

On Friday the National Theatre announced that it would end Shell’s corporate membership at the end of next year, as the arts organisation accelerated plans to make itself carbon neutral in the face of a climate emergency. The move will increase pressure on other cultural institutions, such as the Royal Opera House, British Museum and National Portrait Gallery, all of which have come under pressure over tie-ins with fossil fuel firms.

Mann said: “People needed to know about BP’s sponsorships of arts institutions – they [BP] pretend that they are doing something positive – but the reality of their impact on the world is devastating.”