Mr Byrne had been under pressure after a data breach last month which resulted in information on about 10,000 officers and staff being accidentally disclosed online.
The chief constable of the Police Service of Northern Ireland (PSNI) has resigned following a series of controversies, including a major data breach and the unlawful disciplining of two junior officers.
Simon Byrne had faced a motion of no-confidence in his leadership from the Democratic Unionist Party (DUP).
Its leader Sir Jeffrey Donaldson had said: “Confidence in the chief constable has been eroded, both amongst the wider public but, significantly, also amongst serving PSNI officers and staff.
“In light of that, we believe that a change of leadership is required.”
PSNI chief resigns after vote of no confidence
Mr Byrne has now quit following an emergency meeting of the Northern Ireland Policing Board, which is the oversight body for the PSNI.
He had been under pressure after a data breach last month which resulted in information on about 10,000 officers and staff being accidentally disclosed online.
The breach involved the surnames, initials, rank or grade, work location and departments of all PSNI staff, but did not involve the officers’ and civilians’ private addresses.
The PSNI has confirmed the list is in the hands of dissident republicans, who continue to target officers.
Mr Byrne said he was “deeply sorry” for the “industrial scale breach of data that has gone into the public domain”, describing it as an “unprecedented crisis”.
PSNI: Dissidents have police data
He had on Tuesday been expected to give evidence to the Northern Ireland Affairs Committee which is investigating the breach.
A number of other data breaches have since come to light, including the loss of a police officer’s laptop and notebook which contained details of 42 officers and members of staff after the items fell from a moving vehicle.
Simon Byrne was gaffe-prone – but finding a successor won’t be easy
Policing and politics are never far apart in Northern Ireland.
Simon Byrne was riding the storm over an unprecedented data breach but found himself in the centre of a political hurricane last week.
A High Court judge ruled he had unlawfully disciplined two probationary officers over an arrest because he feared Sinn Fein would withdraw its support for policing.
All three Unionist parties cried foul – accusing the chief Constable of bowing to political pressure from Irish Republicans – and demanded his resignation.
He left a seven-hour meeting of the Policing Board last Thursday insisting he was “not resigning”. Four days is a long time in politics and policing in Northern Ireland.
Simon Byrne was a bit gaffe-prone during his tenure, appearing naive at times about the political sensitives here.
But finding a successor won’t be easy when there is no power-sharing government in place at Stormont to ratify any appointment.
The chief constable’s resignation also comes after a row erupted last week when a High Court judge ruled two junior officers were unlawfully disciplined for an arrest at a Troubles commemoration event in 2021.
The judge, Mr Justice Scoffield, said they had been disciplined to allay a threat that Sinn Fein could withdraw its support for policing.
Sinn Fein has insisted there was no such threat.
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Mr Byrne had said he would not quit following an emergency meeting of the Policing Board last Thursday and indicated he was considering an appeal against the court ruling.
That statement was met by anger from the Police Federation, that represents rank-and-file officers, with its chair Liam Kelly expressing “disbelief and anger” at the chief constable’s remarks.
Mr Byrne had since faced growing pressure, with both rank-and-file officers and civilian staff mulling confidence votes in his leadership.
On Friday, Mr Donaldson confirmed his party had submitted a motion of no-confidence in Mr Byrne to the Policing Board.