Sepp Blatter is to resign as president of world football's governing body Fifa as reports emerge he is under investigation in the United States.
Announcing his shock exit, the 79-year-old Blatter, who has been in power for 17 years, said: "My mandate does not appear to be supported by everybody."
He plans to carry on his duties until a replacement can be elected.
Why stand down now?
Fifa has been dogged by corruption allegations for years now but it was rocked to its core last week by the arrests of seven Fifa officials on charges of racketeering, wire fraud and money laundering.
The charges are part of a US prosecution that indicted a total of 14 people from around the globe.
Further allegations emerged on Tuesday that increased the pressure.
Reports also claimed Fifa general secretary Jerome Valcke was responsible for an alleged payment of a $10m (£6m) bribe in relation to South Africa's bid to host the 2010 World Cup.
The 54-year-old Frenchman denies any wrongdoing.
A separate criminal investigation by Swiss authorities into how the 2018 and 2022 World Cups were allocated is also under way.
BBC Radio 5 live sports news correspondent Richard Conway said Blatter will not have wanted to leave this way.
"He wanted to bring Fifa and football back together but that ignored the huge weight of the allegations that stood against the organisation," said Conway.
"In the end, Blatter's position became untenable and we will perhaps know more in the weeks ahead about what exactly tipped him over the edge."
How did the news break?
The BBC's Kieran Fox was in Zurich when the drama unfolded.
He explains what happened after media were invited to an unscheduled news conference:
"The invitation came at 4pm local. A scramble to Fifa HQ in the leafy hills of Zurich and still no more details.
"The news conference was due to start at 6pm, 6.30pm, then 6.45pm. The large conference room was barely a third full.
"At 6.45pm, a shrugging, forlorn director of communications walked in. Mr Blatter would be making a statement, he said.
"Blatter looked reluctant, perhaps a little sad that 40 years of his life's work was coming to an end.
"The press room was silent. No-one really expected this just four days after winning a fifth term as Fifa president.
"His speech lasted barely four minutes. Speech over, he left to silence."
When will a new president be elected?
Blatter says he wants to bring forward the date of the next Fifa congress so members can elect his successor "as soon as possible".
It is due to take place on 13 May, 2016 in Mexico City but Blatter says waiting until then will "create unnecessary delay".
He added: "I will urge the executive committee to organise an extraordinary congress for the election of my successor at the earliest opportunity.
"This will need to be done in line with Fifa's statutes and we must allow enough time for the best candidates to present themselves and to campaign."
The extraordinary congress is expected to take place between December 2015 and March 2016.
Who wants to be the next Fifa president?
Prince Ali bin al-Hussein was beaten by Blatter in last week's election as the Swiss earned a fifth term in office.
The Jordanian has indicated he is willing to step into the role, insisting he is "always there to serve football".
He added: "I think that's the most important thing and to do so much work to fix this organisation in a proper way."
Michel Platini, president of European football's governing body Uefa, is also being tipped as a likely contender.
The 59-year-old Frenchman did not want to stand against Blatter but this could be his opportunity to make his move.
Former France international David Ginola has also confirmed he intends to stand but his chances of success are slim.
What are Blatter's plans?
Blatter says he stood for re-election as he felt it was the "best option for football". When he quit, he said he did so in Fifa's best interests.
However, he indicated his influence will not end just yet and said his focus now was "driving far-reaching, fundamental reforms that transcend our previous efforts".
He added: "For years, we have worked hard to put in place administrative reforms, but it is plain to me that, while these must continue, they are not enough. We need deep-rooted structural change."
Perhaps of more immediate concern for Blatter is the scope of the twin corruption probe, particularly the US investigation.
US officials quoted in the New York Times said they hoped to gain the co-operation of some Fifa figures now under indictment to try to build a case against the Fifa president.
"His troubles may only just be beginning," added Conway.
How has the football world reacted?
Platini had urged Blatter to quit following the arrests in Switzerland.
After Blatter's decision to step down, Platini said: "It was a difficult decision, a brave decision, and the right decision."
The Football Association has been vociferous in its criticism of Blatter.
News of his exit was welcomed by FA chairman Greg Dyke.
"This is great news for football," he told the BBC. "It should have happened years ago."
He added: "There has to be a root-and-branch investigation of Fifa. It has all got to be transparent in the future."
Former Portugal international Luis Figo, who withdrew from the presidential election eight days before the vote, said Blatter's exit represented "a good day for Fifa and for football".
He added: "Change is finally coming. Now we should, responsibly and calmly, find a consensual solution worldwide in order to start a new era of dynamism, transparency and democracy in Fifa."
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World Cup sponsor Coca-Cola issued a statement calling Blatter's resignation "a positive step".
It added: "We believe this decision will help Fifa transform itself rapidly into a much-needed 21st century structure and institution."
Blatter's daughter, Corinne, told the BBC that she was "both sad and relieved", adding that the pressure "had been wearing him down".
She added: "My wish now is for calm to be restored, both for my father and for world football.
"But, above all, I wish for the world to finally acknowledge the great things he has done for football in the last 40 years.
"My father is my father. He is a wonderful person. By making this decision, he also wants to protect us, his family, first and foremost.
"His decision has nothing, absolutely nothing at all, to do with the accusations that are currently circulating.
"My father is an honest person who has dedicated his life to football."
What of the 2018 and 2022 World Cups?
England lost out to Russia in the bidding for the 2018 World Cup.
Simon Johnson, who led England's bid, told BBC Radio 5 live: "I want the full facts around the bidding to be known and published. I want there to be openness and transparency.
"If everybody won it because of a fair fight and everything was fair and objective and transparent, then good.
"If it was found there was improper behaviour in any way by any of the winning bidders, then Fifa must have a look at whether they should re-open the process."
Johnson said he still thinks the 2018 tournament will go ahead.
"The preliminary draw for the Russia World Cup is a few weeks away and it's too late to change that," he said.
However, he felt Qataris may be worried about losing the 2022 competition, a sentiment echoed by Dyke.
"If I was in Qatar now, I would not be very confident," said Dyke.
Those comments drew a sharp response from Sheikh Hamad bin Khalifa Al Thani, president of the Qatar Football Association.
"We would urge Mr Dyke to let the legal process take its course and concentrate on delivering his promise to build an England team capable of winning the 2022 Fifa World Cup in Qatar," he said.