Sepp Blatter says he will not stand down as Fifa president because of the criminal proceedings opened against him by Swiss investigators.

The Swiss, 79, is suspected of signing a contract that was “unfavourable to Fifa” and making a “disloyal payment” to Uefa president Michel Platini. Blatter, who had already said he will stand down in February 2016, claimed he “had done nothing illegal or improper”.

Platini, 60, has written a letter to Uefa members denying any wrongdoing. In a statement released through his lawyers, Blatter said a £1.5m payment made to Platini, the head of European football’s governing body, in 2011 was “valid compensation and nothing more”. Both men are also facing investigation by Fifa’s independent ethics committee over the payment, which Platini said was for work as Blatter’s technical advisor between 1999 and 2002.

Platini, who was interviewed as a witness by officers from the Swiss attorney general’s office, stressed that the payment had been “fully declared” to the authorities. The Frenchman said he was “aware that these events may harm my image and reputation” and released a statement for “reasons of transparency”.

The 2011 payment came nine years after Platini’s work for Blatter – and two months before Uefa gave its backing to the Swiss before a presidential election. That is something that now must be explained, according to Scottish FA chief executive Stewart Regan.

“It is an essential piece of information that still needs to be provided,” Regan said.