The transfer of Cristiano Ronaldo from Sporting Lisbon to Manchester United in 2003 included a hitherto undisclosed £1m payment to an unnamed agent, believed to be the Italian Giovanni Branchini, the Guardian can reveal.

The payment, which has never been made public before, emerged in the court battle in Porto which Ronaldo's agent, Jorge Mendes, is defending against the English football agency Formation.

Neither United, Branchini nor Mendes responded to questions from the Guardian about what Branchini, a senior agent based in Italy, did to merit payment in the £12.24m deal, when Mendes was Ronaldo's agent. In a defence document Mendes's Gestifute agency has filed with the Porto court, which the Guardian has seen, Gestifute said that it "did not receive any commission" from United directly when Ronaldo joined, but that Gestifute was paid by Branchini. At the time, according to Formation's claim, Mendes told them he had been paid €400,000 as his fee for negotiating Ronaldo's move. The Football Association, which has been required by court orders in Portugal and here to disclose all payments made through it on the Ronaldo deal, has confirmed that €150,000 was paid to Gestifute, while the rest of the £1.129m which United declared it paid in agents' fees went to "another agent".

These details of the landmark deal that took the dazzling 18-year-old to Old Trafford have emerged in the legal action Formation has brought against Gestifute in Porto. The English agency argues in its claim, seen by the Guardian, that it had a two-year partnership agreement with Gestifute, promoting Mendes's client players to English clubs, and that the agents' fees would be split equally.

Formation claims it shared fees after negotiating the first two moves to the UK for Gestifute players, Hugo Viana, to Newcastle, and Nuno Capucho, to Rangers, but that Mendes reneged on the deal once José Mourinho, also his client, went to Chelsea and signed the three Gestifute Portuguese players Ricardo Carvalho, Paulo Ferreira and Tiago Mendes. Formation says in its claim that it agreed to accept £80,000 from Mendes following Ronaldo's signing, rather than half the €400,000 that Mendes said United had paid him, because it wanted to preserve what had become a fruitful partnership.

However, after United disclosed in their 2004 accounts £1.129m paid to agents in the Ronaldo deal, Formation accused Gestifute in its court claim of having "concealed" the full amount of the fee, although it acknowledged that other agents may have been paid too.

Sir Alex Ferguson has always said that United signed Ronaldo after the teenager played mesmerically against United in a friendly with Sporting Lisbon on 6 August 2003 to open the Portuguese club's José Alvalade stadium. Other United sources have told the Guardian the club were planning to sign Ronaldo before that and allow him to mature further at Sporting but, with Arsenal also considering a bid, United snapped him up after his thrilling performance. Ronaldo joined United on 11 August, only five days later, and what duties Branchini performed on the deal, apparently acting for United, is unclear.

In its claim, Formation is asking the court to award it half of whatever fee Mendes's Gestifute agency did in fact receive for the Ronaldo deal, claiming that the signing was made within the two agencies' two-year partnership agreement to share fees equally. In its defence, Gestifute is arguing its partnership with Formation was not a genuine agency agreement that required it to share fees equally and, in relation to Ronaldo, that it was not paid commission by United, but by Branchini. Gestifute in Porto is defending Formation's claim for half the €2.9m Chelsea paid on the Carvalho, Ferreira and Tiago signings by arguing it was not paid anything, and that any fees were paid to Gestifute International Limited, a company registered in Ireland.

Both Branchini and Mendes declined to reply to a detailed series of questions asked by the Guardian, about why Branchini was paid in the deal and why, as claimed by Mendes, he then paid Gestifute. Gestifute did say in a statement about the Formation case: "This is a normal commercial dispute. The matter is being dealt with by our lawyers, and we are very confident that the court will decide on our favour."

United would not comment specifically on the payments the club made, but a spokesman said: "The deal was agreed, the correct payments made and we were delighted to secure Ronaldo who, with his hard work and our coaching, turned into the world's best player."