Ulster Unionist Party leader Doug Beattie has urged the UK government to share details of any proposed deal to restore devolution at Stormont.
It comes as DUP executive members received an invitation to a meeting with DUP leader, Sir Jeffrey Donaldson.
Members have been asked to attend on Monday for a "detailed update" on the current political situation.
The DUP collapsed the executive nearly two years ago in protest against post-Brexit trade arrangements.
Since then efforts to restore power-sharing have seemed close to fruition several times but Stormont has not yet been restored.
The party has been in talks with the UK government seeking changes to the arrangements which were agreed with the EU in a deal called the Windsor Framework.
The DUP has more than 120 executive members, including MLAs, party officers and others.
The email invitation was leaked on X, formerly known as Twitter, on Friday evening.
Responding to the leaked email, Mr Beattie warned the UK government to "stop ignoring those parties with a right to enter a future assembly executive" and to make details of any proposed deal available to all.
"The fact that Sinn Féin have known what the deal contains is also no real shock. However, not knowing if the Alliance Party have been given the details of a deal, I do know that the Ulster Unionist Party has not," he said in a statement on Saturday.
"If the UK government think that as a party, entitled to enter the executive, they can keep us in the dark, they are making a big mistake."
Mr Beattie said the UK government have "badly misread the situation" if they think his party will "simply step into an executive facing a Sinn Féin and DUP carve-up".
"It now sits with the Ulster Unionist Party to consider its position going forward as we watch unelected officials being briefed in detail while elected representatives remain in the dark," he added.
Earlier this week, the deadline for restoring devolution was extended to 8 February; if there is no executive in place by then, Northern Ireland Secretary Chris Heaton-Harris will be under a legal duty to call an election.
It is understood he may instead put new governance arrangements in place including a greater role for London.