President to be questioned on Sunday along with Mohamed bin Hammam and Jack Warner as bribery investigation widens

World football's governing body has been plunged deeper into crisis after its ethics committee widened a bribery investigation to include Sepp Blatter.

The Fifa president will appear in front of the ethics committee on Sunday with Mohamed bin Hammam, his Qatari rival, who is accused of offering cash bribes, and Jack Warner, the Concacaf president who has clung to a pivotal role at Fifa for 28 years despite a string of alleged scandals.

Amid another day of mud-slinging and growing pressure to suspend this week's presidential election, Fifa appears determined to press on with this week's congress of all 208 member nations. Sponsors, including Adidas, have also raised concerns about the impact of the latest and most serious corruption allegations.

As the most senior figures in world football descended on London for tonight's Champions League final, the main topic of conversation was not the clash between Manchester United and Barcelona but the shattered credibility of Fifa.

Blatter was due to attend but it is unclear whether he will make the trip from Zurich in light of the latest allegations. He said: "I cannot comment on the proceedings that have been opened against me. The facts will speak for themselves."

"I think the process is fast descending into a farce," the sports minister Hugh Robertson said. "It's impossible to have a sensible election when both of the candidates have been accused of corruption. The sensible thing would be to suspend the election until these allegations have been properly investigated."

The claims against Blatter mean that 10 of Fifa's 24-man executive committee have now either been found guilty or accused of corruption in the last 12 months.

Fifa's ethics committee was obliged to investigate Blatter under its own code after Bin Hammam claimed the president knew all about alleged payments handed to Caribbean Football Union (CFU) officials at a conference on 10-11 May.

Bin Hammam and Warner had already been summoned to appear before the ethics committee on Sunday after the US executive committee member Chuck Blazer was approached by concerned CFU members and he engaged a Chicago lawyer to conduct an investigation.

The dossier is understood to contain signed affidavits from witnesses, photographs, text messages and emails that claim Bin Hammam and Warner arranged for $40,000 (£24,000) bundles of cash to be dispensed in private meetings to members of the CFU, 25 of which have a vote in Wednesday's election.

Yesterday Bin Hammam claimed the allegations were a "tawdry manoeuvre" to discredit him ahead of the election, claiming for the first time the money was for administrative costs and travel expenses.

"Nobody has ever tried to hide the fact that Mr Bin Hammam paid for the delegates' travel and accommodation expenses and covered the meeting's administrative costs," said a statement, in which he also said there was "growing evidence of a conspiracy" against him.

Blatter, president of Fifa during a 13-year period when its revenues and influence have boomed but allegations of corruption have multiplied, has denied he knew anything about the payments or orchestrated the affair, in typically overblown style.

"To now assume that the present ordeal of my opponent were to fill me with some sort of perverse satisfaction or that this entire matter was somehow masterminded by me is ludicrous and completely reprehensible," he wrote in a column for InsideWorldFootball.

Blatter has privately insisted he knew nothing of Blazer's dossier, compiled by a Chicago lawyer after CFU members who did not accept the cash approached Blazer with their concerns, until he arrived back from Japan on Wednesday.

Even before the latest round of corruption allegations, Robertson has said he plans to talk to fellow sports ministers across the world and European Union partners to put pressure on Fifa to reform.

"Fifa needs to urgently reform in the way that the International Olympic Committee did after Salt Lake City. Sports governing bodies have to be transparent and accountable, and change has to happen."