Sir Alex Ferguson has predicted that Rio Ferdinand will become the best defender in the world following the completion of his £30m transfer to Manchester United this afternoon.

The deal, confirmed by the two clubs in separate statements to the Stock Exchange, makes Ferdinand the most expensive British footballer in history. But Ferguson is convinced the former West Ham and Leeds defender will live up to his price tag.

"At 23 years of age he has great potential," he said. "We are very confident that he will mature here and become the best defender in the world in the next three or four years."

Ferdinand, who has agreed a five-year deal rumoured to be worth £70,000-a-week, admitted it hadn't been easy to leave Leeds, but added: "It's a short career and opportunities like this don't come around all the time.

"I've come in here to improve as a player and fulfil every bit of potential I've got," he continued. "I want win trophies, I want to win things and I feel that I can do it here."

Ferdinand also thanked the Leeds fans for helping him mature in his 20 months at Elland Road, adding: "They were always behind me 100% and the team - and they brought me on definitely as a player."

A relaxed-looking Ferguson went onto admit that the deal had taken long-than-expected to complete. "A transfer like this one had to be difficult simply because Leeds are a big club and one of our fiercest rivals and there were three years left on his contract," he explained.

"But the most important thing was Rio wanted to come here. Things contributed to the deal but it was never going to be easy - and Peter Ridsdale will bear that out. It was a long road to get here today."

Ferguson also revealed that he attempted to sign Ferdinand after seeing him as trainee.

"We tried to buy him when he was 17 - I phoned [then Bournemouth manager] Mel Machin and told him 'I'm about to give you some money for Rio Ferdinand' but he told me that Rio was on loan from West Ham. We made an enquiry but got a straight no.

"We also had the opportunity to sign him when he went to Leeds but the timing wasn't right for us - Ronnie Johnsen and Jaap Stam were established in central defence, while Wes Brown was coming through as well.

"But we are so delighted we've signed him now," he continued. "He's going to be a fantastic player. Who wouldn't want him at Old Trafford? Everybody wanted him here."

United have agreed to pay Leeds £15m up front for Ferdinand, and a further £14.3m in July 2003 - and that fee could rise to £33.3m depending on appearances and United's success over the next few years.

The fee surpasses the British transfer record - set when Juan Sebastian Veron moved to United from Lazio for £28.1m last season - as well as the world transfer record for a defender, set when Lilian Thuram moved from Parma to Juventus for £26m last season.