House of Commons Speaker John Bercow has ruled that Gerry Adams has been disqualified from Parliament.
Earlier, Downing Street had apologised to Mr Adams after the prime minister said he had accepted a Crown title.
David Cameron told the House of Commons Mr Adams had accepted the title in order to resign his Westminster seat.
This was disputed by the Sinn Fein president who said he had not applied and had received an apology from the PM's office.
Parliamentary rules mean MPs cannot officially resign and have to accept a crown office to give up their seat.
Following a point of order from Labour's Thomas Docherty in the House of Commons on Wednesday night, Mr Bercow said: "I can inform the House that I have received formal notification from the Chancellor of the Exchequer that Gerard Adams has been appointed to be Steward and Bailiff of the Manor of Northstead."
He said that under the Disqualification Act 1975, Mr Adams was "therefore disqualified from membership of the House".
Earlier, a Treasury spokesman said Mr Adams had said "publicly that he is resigning from Parliament".
"Consistent with long-standing precedent, the Chancellor has taken this as a request to be appointed the Steward and Bailiff of the Manor of Northstead and granted the office," he added.
David Cameron had told MPs that Mr Adams had accepted the role.
Speaking in response to a question from DUP MP Nigel Dodds, the prime minister said he was glad that the rules had been followed.
To laughter from MPs, he added: "I'm not sure that Gerry Adams will be delighted to be Baron of the Manor of Northstead. But nonetheless I'm pleased that tradition has been maintained."
Later on Wednesday, Mr Adams said that when he was told of Mr Cameron's remarks it was the first he had "heard of this development".
In a statement he said the claim that he had accepted a crown title was "untrue" and that he had "simply resigned"
"I am an Irish republican," he said.
"I have had no truck whatsoever with these antiquated and quite bizarre aspects of the British parliamentary system."
He described Mr Cameron's announcement as "bizarre" .
"I am sure the burghers of that Manor are as bemused as me," he added.
"I have spoken to the Prime Minister's private secretary today and he has apologised for today's events.
"The onus is on the Westminster parties to call a by-election as soon as possible in the West Belfast constituency."
On Tuesday, the Speaker's Office had told the BBC that it was its understanding that Mr Adams had not applied for the crown position and therefore remained an MP.