Sir Alex Ferguson has declared Manchester United's Barclays Premier League showdown with Arsenal on Sunday as his club's "biggest game of the season". Although United had business of their own to attend to on Wednesday as they sealed a Carling Cup final berth at the expense of Manchester City, it did not escape Ferguson's notice that Chelsea swept back to the Premier League summit with their own win over Birmingham, and still have a match in hand. So, with the Reds' margin for error reducing all the time, Ferguson knows the importance of beating the only other side he believes can deprive United of a record fourth consecutive title. "Looking at the way the league is shaping up, this game on Sunday is the biggest of the season for us," said Ferguson. "If Chelsea win their game in hand they go four points ahead of us. That is the catch-up part. "But Sunday's game has great relevance because both sides have a genuine chance of winning the league." United had never won at the Emirates Stadium until last season's Champions League semi-final. Encounter That one-sided encounter changed the run, since when Arsenal have suffered successive heavy home defeats to Chelsea, the latter occasion in November providing the trigger for a number of the Gunners' critics to write them off completely. "It is amazing how this league has changed," said Ferguson. "As I have said all along, the team that is most consistent will win the league now. "Arsenal's current form has been very good. It has brought them back into the picture. That is why this is such a massive game for us." What this season has done is revive a history that goes back 20 years, when a thrilling last day title win for Arsenal at Liverpool was followed 12 months later by Ferguson winning his first trophy for the Old Trafford outfit. The rivalry has endured ever since. "These games with Arsenal are historic," he said. "Since I came down here, apart from the early years before George Graham won his first title, it was Arsenal and United who were mostly competing for the trophies. "Without doubt Chelsea have become a prominent team in that respect too and it has been ourselves and Chelsea challenging for the league in the last few years. "But you can never dismiss Arsenal because their history will always guarantee a challenge. And they are definitely in the fray again." Where will the game be won and lost? Have your say.