• Successor will be chosen at extraordinary general meeting of Fifa • Blatter to stay until next president is elected between December and March • Reports in US say Blatter being investigated as part of corruption probe

Sepp Blatter has dramatically quit as Fifa president, days after he was defiant in re-election for a fifth term and sparking a flurry of speculation over the future of world football and the fate of the next two World Cups in Russia and Qatar.

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Under intense pressure from ongoing investigations by the FBI and Swiss prosecutors that have already led to 18 senior football executives being charged in the US on charges of money laundering, tax evasion and racketeering, Blatter said he had decided to step down.

But the 79-year-old Swiss, who defeated his Jordanian challenger Prince Ali bin Al-Hussein on Friday despite the mounting crisis, said he would stay on for at least six months to allow time for a proper election to replace him between December 2015 and March next year.

Hours after Blatter stepped down there were reports in US media that he was being investigated by US authorities as part of their corruption inquiries.

Blatter, who has previously pressed on through an endless series of corruption allegations, told the press conference: “While I have a mandate from the membership of Fifa, I do not feel that I have a mandate from the entire world of football – the fans, the players, the clubs, the people who live, breathe and love football as much as we all do at Fifa.

“Therefore I have decided to lay down my mandate at an extraordinary elective Congress. I will continue to exercise my functions as Fifa president until that election.”

In the same press conference room where he had on Saturday faced down questions over Fifa’s culture of corruption and his knowledge of a $10m bribe alleged by US prosecutors to have been routed to the former Concacaf president Jack Warner, Blatter cut a depleted figure.

“Since I shall not be a candidate, and am therefore now free from the constraints that elections inevitably impose, I shall be able to focus on driving far-reaching, fundamental reforms that transcend our previous efforts,” he claimed, complaining that he did not have enough control over those that sit around his executive committee table.

Blatter was said to have spent the weekend since his election victory, following which he blamed attempts to oust him on bitterness from the Americans and English for losing their World Cup bids, listening to pleas to stand down from those closest to him.

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Blatter aide Walter Gagg told AP that Blatter’s mood had shifted over the 24 hours before his announcement. “We know that the in the last 48 hours he was thinking of the future and perhaps what happened in the last hours, this gave him the conviction,” said Gagg, a long-time confidant of Blatter “We had lunch with him yesterday [Monday]. He was relaxed, he was fine,” he said. “I had a very good meeting with him early in the morning [Tuesday]. Then came the different information from the US with this and that.”

Meanwhile the net continued to close in on some of those senior Fifa executives with whom he had been inextricably linked down 40 years at Fifa, 17 of them as president.

They included not only those arrested last week but the Brazilian Ricardo Teixeira, now the subject of a new Brazilian probe over $150m in missing money from last summer’s World Cup, and his long time general secretary Jérôme Valcke.

The day had begun with Valcke facing questions over a letter that proved he had knowledge of a $10m payment to Warner which US investigators allege was a bribe.

Those who had in recent days called on Blatter to stand down in light of the damning evidence outlined by the US attorney general Loretta Lynch and warnings from investigators that further indictments would follow were jubilant.

“I think it is long overdue but it is good news for world football,” said the FA chairman, Greg Dyke, who has been among the most vocal of those calling on Blatter to go.

“It now means that we can get someone in to run Fifa, we can get in there and find out where all the money has gone over all these years and sort it out for the future.”

Uefa’s president, Michel Platini, a one time ally of the Fifa president who declined to stand against him but made a personal plea to him to stand down before the election, said: “It was a difficult decision, a brave decision and the right decision”.

Platini is among those who will now be touted as potential replacements for Blatter, along with others inside Fifa such as the increasingly influential Kuwaiti Sheikh Ahmad Al-Fahad Al-Sabah. Prince Ali, defeated by a margin of 60 votes on Friday, refused to rule out standing again.

The Uefa president, who was recently re-elected for another four-year term, will meet others in Berlin this weekend before the Champions League final to start to plot his next move.

But many will call for a new broom untainted by Fifa’s many scandals. And beyond Europe and America there will be disdain for the way in which they perceive the US and the British media pushed Blatter from office.

The Russian president, Vladimir Putin, a big supporter, has already railed against what he sees as a US plot and Blatter retains widespread support in Africa and parts of Asia.

The British Conservative MP Damian Collins, a key mover in the NewFifaNow pressure group, said Blatter might yet have to stand down before next year if more serious allegations emerged.

“What we have to make sure is that this isn’t some sort of palace coup whereby powerbrokers of Fifa are hoping to usher Sepp Blatter out but retain power and control for themselves,” he said. “What we wouldn’t want to see is a group of Sepp Blatter’s cronies leading the power for reform in order to protect themselves.”

The US Justice Department would not comment on Blatter’s announcement, while the Swiss attorney general said Blatter was not under investigation in Switzerland despite an ongoing probe into the 2018 and 2022 World Cups.

Attention will also immediately turn to the already endlessly controversial question of whether the World Cup should take place in Qatar in November 2022. With stadiums already under construction in Russia for the 2018 tournament, it would pose a huge logistical challenge to throw that tournament into doubt.

Construction of stadiums has also begun in Qatar, amid pressing human rights issues and unanswered questions over how it won the bid. But a new president could yet decide to re-run the vote if an ongoing Swiss criminal investigation, conducted in parallel to the FBI probe, finds new evidence of bribery.