Arsene Wenger was left surprised after his Arsenal team qualified for the last 16 of the Champions League ahead of Manchester City, Chelsea and Manchester United.

The Gunners booked their place in the knock-out stages of the Champions League for the 12th consecutive season when they beat Borussia Dortmund 2-1 at the Emirates Stadium on Wednesday.

It seemed doubtful that the Londoners would even qualify for the group stages of the competition after struggling against Udinese in the play-offs, at a time when the club’s form was so poor that some fans were even calling for Wenger’s head.

At that time, City, United and Chelsea were on top form, but Wenger has revitalised his team, who have gone on a nine-match unbeaten run that has moved the club to within touching distance of the top four.

The Frenchman never expected the Gunners to come through their poor patch in such a clinical fashion and expressed his surprise that Arsenal’s English rivals in Europe had not already booked their place in the second round.

“I am surprised (that we are the first English team to qualify),” Wenger said.

“Manchester United could have qualified (by beating Benfica on Tuesday) and of course everyone expects Man. City to qualify. We were in a difficult group and I am very happy to see where we have come from.

“When we drew Udinese in the qualifier, you couldn’t bet exactly that we would go through like we have done so overall I am happy. We came through a very difficult period because we were united and we worked hard together.”

Predictably, it was Robin van Persie who proved the architect of Dortmund’s downfall. The Dutchman headed home from Alex Song’s cross to put Arsenal ahead just after the break and added a second four minutes from time before Shinji Kagawa pulled back an injury-time consolation for the Germans.

Van Persie has now scored a staggering 33 goals in 30 games for Arsenal in 2011 and Wenger hopes his captain will continue his excellent form in the coming weeks.

The Arsenal boss said: “At the moment Robin is an exceptional player. He takes advantage of our offensive style. At the moment he is the one who scores all the goals so we hope we can keep him fit and with us for a long time.”

The defeat leaves Dortmund bottom of Group F, with their hopes of qualifying pinned on beating Marseille and Arsenal overcoming Olympiakos.