Theresa May has been "empty-chaired" by organisers of a politicians' round-table with the Swedish teenager behind a global wave of youth climate change protests.
Sixteen-year-old Greta Thunberg met Westminster party leaders including Jeremy Corbyn as Extinction Rebellion protesters gathered outside in Parliament Square.
Organisers of the meeting pointedly left a place at the table for the Prime Minister, who was chairing Cabinet in 10 Downing Street at the time.
They said that no response had been received to an invitation issued to Mrs May to attend.
Earlier, the PM's official spokesman said he was "not aware" of any request for her to meet Miss Thunberg.
The teenager won worldwide fame after beginning a solitary School Strike for the Climate protest outside the Swedish Parliament in August last year.
Her stand inspired school walkouts around the world, with hundreds of thousands of young people in countries including the UK joining the Global Climate Strike in March.
Swedish climate acticist Greta Thunberg meets leaders of the UK political parties at the House of Commons in Westminster.
Along with Mr Corbyn, Miss Thunberg met Liberal Democrat Sir Vince Cable, Green MP Caroline Lucas and the Westminster leaders of the Scottish National Party and Plaid Cymru, Ian Blackford and Liz Saville Roberts.
Opening the round-table talks, the 16-year-old activist told the MPs: "We just want people to listen to the science."
Caroline Lucas told Miss Thunberg: "I hope this will be the beginning of an ongoing dialogue with young people.
"I think all of us are really committed to trying to ensure that there's an ongoing way to really make sure that all of our policies and all of our work in Parliament is properly scrutinised by young people with a perspective on climate in particular.
"We want young people's voices to be heard in Parliament. This is such an important moment, when the Inter-Governmental Panel on Climate Change say we have just 11 years left to get off the collision course we are on for climate catastrophe.
"That sense of urgency is here as never before, at a time when there have been protests on the streets not just of London but around the country and all the different nations of the UK. More than ever, politicians have our ears open to your message."
Mr Corbyn told Miss Thunberg: "Well done for what you have done."