Manchester United forward Wayne Rooney lacks the mentality to become a "world-class" player, says Queens Park Rangers midfielder Joey Barton.
Rooney, 28, recently signed a new contract that puts him among the highest-paid players in football.
But Barton - an admirer of Rooney - said his mental approach leaves him behind the world's best on the pitch.
"Look at his approach and mentality compared to Cristiano Ronaldo's. Polar opposites," he told BBC Radio 5 live.
The 31-year-old added: "Rooney potentially, could have been world class. But why is Ronaldo a Ballon d'Or winner and Rooney isn't?
"Players who have worked with both say the level of discipline Ronaldo has is the reason he is the best player in the world."
Barton stressed that although he considers Rooney to be "the epitome of a class Englishman", he feels players such as Real Madrid forward Ronaldo are more deserving of the 'world class' label.
"When you see Rooney smoking cigarettes," Barton continued, "is he doing everything he can to be the best player in the world? The gulf is getting wider but if it was on talent alone, Rooney is a better player than him."
Since making his senior debut for Everton at the age of 16 in August 2002, Rooney has gone on to win five Premier League titles, two League Cups, three FA Community Shields and the Champions League and Fifa World Club Cup once with United, who he joined for £25.6m in 2004.
But Barton said he believes Rooney falling short of his potential was part of a wider problem within English football, adding that Roy Hodgson's England side have "no chance at the World Cup," and that "no-one believes we can win it - the manager, the media, the players."
The former Manchester City and Newcastle player also added that he "would have sacked Greg Dyke on the spot" for a cut-throat gesture the FA chairman made as England were placed in Group D alongside Italy, Uruguay and Costa Rica during the draw in Brazil on 6 December.