Party leaders will be taking part in a televised debate this evening on ITV ahead of the general election on June 8.
The event will see Liberal Democrat leader Tim Farron, Ukip's Paul Nuttall and the SNP's Nicola Sturgeon lock horns.
Plaid Cymru's Leanne Wood and Green co-leader Caroline Lucas will also take part in the two-hour show being broadcast from Salford.
The Prime Minister has refused to take part in TV debates and a spokesperson for Labour leader Jeremy Corbyn has said he would not participate without his main rival being there.
The broadcaster has said the invitation to take part remains open until the programme starts at 8pm, but if they do not show up they will not face the ignominy of being "empty chaired" as ITV said the stage will have "the right number of podiums for leaders who attend on the night".
Ms Lucas said on Twitter:
Ms Wood said: "The Prime Minister and the Labour leader's absence will no doubt cast a shadow over the event. Their refusal to take part is inexcusable.
"They are denying people access to the widest possible range of opinion. It's not good for democracy."
Mr Corbyn issued a direct challenge to the Prime Minister to take part in a debate during an ITV News Facebook Live Q&A session with voters on Monday.
In his challenge, read out by ITV News Political Editor Robert Peston, the Labour leader said: "Hello Theresa May, as Prime Minister you've served your elite friends by giving them tax cuts when wages have stagnated, house building is at its lowest since the 1920s, there are 20,000 fewer police on our streets since 2010 and the NHS is in crisis. "Do you not think the British people deserve to see me and you debate live and on television?"
Mrs May responded: "What I think is more important is actually that I take questions directly from the voters.
"I don't think people get much out of seeing politicians having a go at each other, I think people want to hear directly."