Sinn Féin has said it will not attend talks with Foreign Secretary James Cleverly because its party leader Mary Lou McDonald has been "excluded".
Mr Cleverly and Northern Ireland secretary Chris Heaton-Harris were due to meet Stormont party leaders in Belfast to discuss Brexit's Northern Ireland Protocol and the collapse of devolution.
Ms McDonald said her exclusion from the meeting was "absolutely bizarre and unacceptable".
Read more: Protocol 'risk' to NI's place in UK must be addressed says Cleverly
The UK government said the meeting was "for Northern Ireland politicians" and Sinn Féin's leader in the Assembly, party vice-president Michelle O'Neill, had been invited.
Unionists suggested it was a diplomatic issue that Mr Cleverly could not meet Ms McDonald - leader of the opposition in the Republic - ahead of meeting his Irish government counterpart, Tánaiste Micheál Martin.
The SDLP also withdrew from the round-table discussion, saying it would not attend unless Ms McDonald was allowed to take part.
The meeting went ahead with only DUP, UUP and Alliance representatives present.
Speaking in Belfast, Ms McDonald said: "We had a chance this morning to mark progress, to exchange views, to be constructive, to work together, to listen to each other.
"But rather than having that kind of meeting, a kind of British Tory petulance has emerged. I think that is a terrible shame."
She added: "We need respect, we need inclusion, we need engagement, we need dialogue."
A UK government spokesperson said: "This meeting is for Northern Ireland politicians to talk through issues around the Northern Ireland Protocol with SOSNI and the UK Foreign Secretary.
"The leader of Sinn Féin in the Assembly was invited and remains invited. Her attendance is a matter for Sinn Féin but she was not excluded."
Speaking following the meeting, DUP leader Sir Jeffrey Donaldson said he did not want to get drawn into the dispute.
He said: "My understanding is the NIO (Northern Ireland Office) didn't exclude any party. 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.
"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."
Sir Jeffrey said he believed Mr Cleverly recognised that any deal on the protocol had to be acceptable to unionists.
Stormont has been in limbo since last February when the DUP withdrew its First Minister from the Executive in protest against the Irish Sea trade deal.
A agreement earlier this week between the UK and European Union on post-Brexit data-sharing has been hailed as a positive step in ongoing talks to resolve the impasse.
Alliance MLA Andrew Muir called for a "quickening of the pace" in negotiations.
"It has been going on for a number of months now and it is important that that is brought to a conclusion that is a durable solution but also protects Northern Ireland’s place within the single market," he said.
Ulster Unionist leader Doug Beattie said he pushed for more input from local politicians.
He said: "We need to be in a position that if they are going to agree something, we have the ability to say that is not going to work.
"That is a red flag and I made that point quite strongly to the Foreign Secretary."
Mr Cleverly and Mr Heaton-Harris were also due to meet business groups and visit Saintfield Garden Centre and Nursery as part of discussions on the protocol.
Ahead of his visit, the Foreign Secretary said issues with protocol which "risk and undermine" Northern Ireland's place within the UK must be addressed.
On Thursday, Stormont party leaders are also due to hold separate talks with Taoiseach Leo Varadkar and Labour Party leader Keir Starmer.
The meetings come ahead of a looming deadline to restore devolved government at Stormont.
If a new Executive is not formed by January 19, the UK government assumes a legal responsibility to call a fresh Assembly election by April 13.
The government introduced legislation to extend the timeframe for forming an Executive after a previous deadline of October 28 was missed.
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