Arsenal had been overwhelming favourites to end their six-year trophy drought but a mix-up between defender Laurent Koscielny and goalkeeper Wojciech Szczesny allowed Martins to secure Birmingham’s first major trophy since 1963.
Birmingham, battling against relegation from the Premier League, led with a header from towering striker Nikola Zigic but Arsenal equalised through Dutchman Robin van Persie and looked poised to win the first of the four trophies they are seeking this season.
“Relatively speaking, it is the greatest moment of my career, in terms of personal achievement, when you consider the team we were up against,” Birmingham manager Alex McLeish told reporters.
Arsenal manager Arsene Wenger defended his 20-year-old goalkeeper Szczesny. “I am bitterly disappointed. I hope this won’t have a negative affect on our season,” Wenger said. “You have to be positive because he is a young boy,” he added. “When we conceded the goal there was no time for us to respond.”
Arsenal, who beat Barcelona in the Champions League last week and lie second in the Premier League table, had an early let-off when Zigic played Lee Bowyer through and he was wrongly flagged for offside.
Nasri set up Andrei Arshavin for Arsenal’s first real scoring chance when the stocky forward had time to turn and shoot and Ben Foster did well to save with his feet.
Birmingham, with 6ft, 8ins tall Zigic as their target man, tended to go for the high ball and the forward’s aerial power brought the opener after 28 minutes when he outjumped Szczesny to head his side in front.
Arsenal responded when Van Persie headed over the bar but he was right on target 11 minutes later. Wilshere unleashed a ferocious left-foot shot from the edge of the penalty area which smashed against the bar. Birmingham failed to clear the danger and the ball came out to Arshavin who found Van Persie with a perfectly weighted cross for the Dutchman to volley crisply home.
Arsenal looked the more likely winners in the second half but with extra time looming Zigic flicked the ball on and Nigerian Martins, just six minutes after coming on to the pitch, took advantage of miscommunication in the Arsenal defence to stroke it into an empty net.
