Matt Prater's last-gasp field goal capped a remarkable comeback as the Detroit Lions wiped out a 21-0 half-time deficit to beat the Atlanta Falcons 22-21, and leave Falcons coach Mike Smith fuming at Wembley.

When the unfancied Falcons stormed into a 21-0 half-time lead, it appeared a one-sided affair was on the cards.

But Detroit had other ideas and gave themselves one final drive to win it inside the final two minutes, and win it they did but only after profiting from a late penalty call against them.

Prater missed his first attempt from 48 yards but the Lions had drawn a flag for delaying the game, giving him a second go which he did not pass up.

The Falcons arrived in London with a 2-5 record having lost four in a row, while the Lions were sitting proudly atop the NFC North division at 5-2.

As had been expected, Detroit's star offensive duo Calvin Johnson and Reggie Bush were ruled out before kick-off, but most still expected their premium defence to overwhelm Atlanta.

Such expectations were blown away in the opening minutes of the game as Matt Ryan led the Falcons on an 80-yard drive that ended with a seven-yard pass to Devonta Freeman and a touchdown.

While the Lions struggled to get going themselves, they offered a helping hand for Atlanta, with a pass interference call against Darius Slay putting Ryan in position to find Bear Pascoe from one yard for a 14-0 lead.

The Lions might have worried it was not their day when Rashean Mathis intercepted Ryan in the end zone and ran it back 102 yards for the touchdown only to have it waved off for a foul.

Atlanta punished that mistake moments later when Steven Jackson ran in from one yard for their third touchdown and a 21-0 lead at half-time.

But momentum can swing quickly in the NFL, and Wembley saw just how quickly after the break.

The change came gradually, with a promising drive yielding no more than a field goal for Detroit early in the third quarter.

But they had points on the board and better was to come, with quarterback Matt Stafford connecting with Golden Tate for a 59-yard touchdown bomb that brought the game to life.

Now it was Atlanta's turn to help their opponents and an inexplicable pass from Ryan turned into a gift-wrapped interception for Cassius Vaughan who ran back 45 yards to put Detroit in scoring position.

They again settled for a field goal but the Falcons were rattled and Detroit drove forward again in the fourth quarter before Stafford picked out Theo Riddick in the end zone.

They missed on a two-point conversion attempt that would have tied the game, but Atlanta were struggling to put the game away and the ball was back in Stafford's hands on his own seven-yard line with 1:38 to go.

He ripped off big passes to Tate and Riddick to cross the half-way line, and they inched their way into field goal range before a huge penalty call against gifted them extra yards.

It appeared Prater had refused the gift as his first kick drifted wide but a flag was already down for delay of game, and given a second chance, Prater snatched victory away from Atlanta.