Barca had recovered from conceding the fifth quickest ever goal in Europe's elite club competition, scored by Alexandre Pato after 24 seconds, through strikes from Pedro and David Villa.
Once ahead they quickly settled into a rhythm, enjoying almost 70 percent of possession, but failed to kill off the seven-times champions and surrendered the lead for a second consecutive game following Saturday's 2-2 La Liga draw at Real Sociedad.
Their disappointment was compounded by the news that midfielder Andres Iniesta will be out for around four weeks after tearing a leg muscle shortly before half-time.
"It wasn't easy trying to penetrate the opponent's penalty area crowded with eight defenders," Barca coach Pep Guardiola told a news conference.
"We tried but at the end they levelled from a corner. What Milan did has a lot of merit. They made it as far as our goal two-and-a-half times and they came away with a point."
SAVIOUR MESSI
Around 90,000 home fans packed into a sultry Nou Camp were stunned into near-silence seconds after the start when Brazil striker Pato left the home defence for dead and beat goalkeeper Victor Valdes with a low shot.
With captain Carles Puyol only declared fit this week after knee surgery and his partner in central defence Gerard Pique sidelined, Guardiola deployed midfielders Sergio Busquets and Javier Mascherano at the back.
The decision immediately appeared to have backfired when pacy Brazil striker Pato exposed their obvious lack of pace.
Straightaway Barca created a slew of chances for an equaliser before World Player of the Year Lionel Messi, after striking a curling freekick against the post, once again came to the rescue.
The Argentina forward, top scorer in the competition the past three seasons, danced through the Milan defence to the byline and squared for Pedro to rifle home with goalkeeper Christian Abbiati stranded.
Barca had recovered their customary poise and a goal had long been coming when Villa stepped up to crack the ball into the top corner from 30 metres.
The home side were in total control without really threatening to score a third until fullback Eric Abidal conceded a stoppage-time corner, although the France international claimed he had been fouled.
Dutch midfielder Clarence Seedorf swung the ball over for Silva to claim an unlikely point.
"When they score an equaliser in the last minute you are left looking a bit stupid," Barca's Brazil full-back Daniel Alves told Spanish television. "We have to pick ourselves up and keep working."
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Milan were missing former Barca striker Zlatan Ibrahimovic, injured in training on Monday, among others, and coach Massimiliano Allegri opted to use Pato and Antonio Cassano in a two-pronged attack.
Pato threatened to score a second several times during the first half hour before
