It would not have been wise to invite Sinn Fein leader Mary Lou McDonald to a political meeting about the Northern Ireland Protocol because she is a "representative of a parliament in an EU member state", Northern Ireland Secretary Chris Heaton-Harris has said.
The row over the exclusion of the Sinn Fein president from a political meeting with the Foreign Secretary rumbled on as Irish premier Leo Varadkar, Tanaiste Micheal Martin and UK Labour leader Sir Keir Starmer were all holding talks in Northern Ireland as part of efforts to resolve the dispute over the post-Brexit trading arrangements.
The Taoiseach and Sir Keir are meeting with the main Stormont parties to discuss the deadlock over the protocol, which the DUP has cited as its reason for boycotting Northern Ireland's devolved institutions since May.
Read more: Sinn Fein withdraws from talks after party leader 'excluded'
Sir Keir began his two-day visit to Northern Ireland by meeting business leaders in Belfast before travelling to Stormont. Mr Martin held talks with Mr Heaton-Harris in Hillsborough.
However, the row over Mrs McDonald's exclusion, which led to Sinn Fein and the SDLP refusing to join roundtable talks with Foreign Secretary James Cleverly and Mr Heaton-Harris earlier this week, has continued to overshadow the meetings.
Asked about it following a meeting with Mr Martin on Thursday, Mr Heaton-Harris said: "Sinn Fein were invited, it is a shame Michelle (O'Neill) didn't come along because it was an update on the protocol discussions.
"There are many factors which go into the thought process. One, to be quite frank, is that the UK Government is negotiating with the European Union.
"We wanted to update Northern Ireland parties on that negotiation and, with the greatest of respect, Mary Lou is a representative of a parliament in an EU member state.
"That might not have been seen as a wise thing to do."
Mr Heaton-Harris said he had extended an invite to dinner to Mrs McDonald.
The Sinn Fein president, who is the leader of the opposition in the Republic, confirmed she has written to Prime Minister Rishi Sunak over the issue.
Speaking on Thursday, Mrs McDonald said: "All of us know that the way that we make progress is together, that's how this works, that's what the history of the last 25 years reflects, so any idea of excluding anybody, excluding the leader of any party, needs to be scotched and knocked on the head now."
One of the reasons given for the failure to invite Mrs McDonald to the talks was that Mr Cleverly could not meet with her until he had met his counterpart in Ireland, Mr Martin.
Asked if he would have had any issue with Mr Cleverly meeting with Mrs McDonald, Mr Martin said: "We certainly got no heads-up in relation to that at all, and I think it needs to be stated.
"I would have had no difficulty that Mary Lou McDonald was at that meeting, but that's a matter for the Foreign Secretary."
Hopes of a deal over the contentious protocol were raised this week when the EU and UK reached agreement on sharing customs data.
Mr Heaton-Harris said: "There has been a tiny bit of progress made in talks with the European Union.
"There is still a way to go, but we are talking in good faith, and as I stand here now there are talks going on, so let's see where they get to.
"We have another assessment next Monday where I will be with the Foreign Secretary and Maros Sefcovic going through the progress that has been made this week."
Mrs McDonald said her party had a "very constructive" meeting with Taoiseach Mr Varadkar in Belfast.
"We have reflected our absolute determination that government must be restored here in the north ... it's unacceptable that we stagger on without an executive," she told media.
"We have also shared our strong view that a deal on the protocol is possible ... and we believe that the window we now have has to be grasped with both hands."
But DUP leader Sir Jeffrey Donaldson said he believed a deal was still some way off.
He told the BBC: "At the moment, while some progress has been made on some technical issues, there are major political issues in those negotiations that have not yet been addressed.
"I don't think we are anywhere close to a deal.
"That was clear yesterday from James Cleverly's report to the political parties that we aren't close to a deal at this stage.
"There is still a lot of ground to be covered before we get to that point."
The protocol has become a contentious political issue, with the DUP refusing to engage with the powersharing institutions until it is dramatically altered or removed.
Unionists oppose the trade barriers it has created between Northern Ireland and the rest of the UK.
For the latest updates and reactions to today's talks follow our live updates here.
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