Mary Lou McDonald said she has written to the British Prime Minister following a decision to exclude her from a meeting with Foreign Secretary James Cleverly.
Renewed optimism around potential progress in restoring the power-sharing institutions quickly evaporated yesterday morning as two of the five parties invited to the talks at Erskine House in Belfast city centre withdrew.
Mrs McDonald, who is in Belfast to meet Taoiseach Leo Varadkar and Labour leader Sir Keir Starmer, also rejected claims from the British Government that there was an issue around the “protocol” of meeting the Sinn Féin leader.
“It was a bad move, it was a bad decision,” Mrs McDonald said on Thursday morning.
“I’ve written to the British Prime Minister (Rishi Sunak) to air my concerns and I can only hope that lessons will be learned and we don’t have a repeat of this distraction politics which was most unhelpful.
“But we crack on today and we get some work done.
“The whole basis of progress in the North is always on the basis of mutual respect, inclusion and recognising people’s electoral mandates. That’s how it works. All of us who work closely in this know that.
“I would have reacted exactly the same way had another leader of another political party been excluded. It’s not acceptable. It’s not the way we do business.”
The SDLP's Matthew O'Toole described the barring of Mrs McDonald as a "surreal farce" and he refused to take part in the meeting chaired by British Foreign Secretary James Cleverly and Secretary of State Chris Heaton-Harris.
The Sinn Féin president branded her exclusion "absolutely bizarre and unacceptable".
She said it was important to recognise every party's mandate.
"We had a chance this morning to mark progress, to exchange views, to be constructive, to work together, to listen to each other," she said.
"But rather than having that kind of meeting, a kind of British Tory petulance has emerged. I think that is a terrible shame."
The British government failed to fully explain the basis for refusing to meet Mrs McDonald. In a statement it said the meeting was "for Northern Ireland politicians" and that Ms O'Neill had been invited.
It was reported that the Sinn Féin president's role as leader of the opposition in the Dáil meant her engagement with Mr Cleverly ahead of a meeting with his counterpart in the Republic was not in line with protocol.
A spokesperson for the Irish government told The Irish News that Dublin had not been consulted about the meeting's attendees.
"We would not have had any difficulties with such a meeting going ahead," the spokesperson said.
Speaking yesterday afternoon, Mr Cleverly said the morning meeting was "to meet with the elected representatives of the people of Northern Ireland".
"I will of course be going to Ireland (sic) in the near future and I'll be meeting Irish politicians, but I very much wanted to hear from representatives of Northern Ireland," he said.
DUP leader Sir Jeffrey Donaldson said he did not want to get drawn into the dispute.
"There was an issue around the protocol of Mary Lou McDonald meeting the foreign secretary ahead of the foreign secretary meeting his counterpart in Dublin," he said.
"That is not a matter for me. I am not going to get drawn into this. It is better when all parties are at the table."
Alliance MLA Andrew Muir said the talks had been "completely overshadowed by the needless decision of the UK government to attempt to dictate who attended".
"It should be up to individual parties to decide who goes on their behalf, with this distraction taking away from what we should have been focusing on," he said.
UUP leader Doug Beattie said he used the meeting to pushed for more input from Stormont politicians in the protocol talks between the EU and the UK.
Mr O'Toole said: "People will look at what happened with this meeting in despair – it’s time for everyone to wise up and work together to fix this mess."