Hamilton took a surprise lead through Richard Offiong early on at New Douglas Park but were pegged back when Celtic levelled thanks to a dubious penalty decision.

Television pictures show Martin Canning's challenge on Cillian Sheridan took place outside the box.

But referee Steve Conroy took the advice of his assistant Chris Young and pointed to the spot before dismissing the Hamilton player.

Shunsuke Nakamura converted from 12 yards before Hartley helped himself to the decisive goal with just four minutes to go.

Match officials will no doubt find themselves under the spotlight again after another high-profile blunder but Hartley believes it was a tough one to call.

"I thought it was a penalty at the time but someone said it wasn't, that it was just outside the box," he said.

"Sheridan looked as though he fell about 10 yards inside the box but they say he was pulled outside it.

"It was a tough decision on Hamilton but there's not a lot we can do about it and thankfully Naka stuck away the penalty."

Rock-bottom Accies had looked set to hold on for a point which could eventually make all the difference in their fight for survival, with the relegation issue looking set to go right to the wire this season.

But Hartley was thrilled with his strike, which allowed Celtic to restore their four-point advantage at the summit of the SPL, following Rangers' own win over St Mirren.

"It was a crucial time," he admitted.

"It was the last few minutes of the game and I played a one-two with Scott Brown and managed to strike it very well. Thankfully it went in."

Offiong had no complaints about Canning seeing red but did feel the penalty award was harsh, admitting: "It was a big blow, it would have been good to go in at half-time still 1-0 up.

"I thought it was outside the box but he is last man and the letter of the law says he has to go so we can't really complain about that.

"We don't seem to be getting any luck at the moment but hopefully these things will even themselves out over the season."

Rather than be despondent about the result, Offiong is determined to take confidence from the many positives.

"We are devastated but we will take a lot from the game," he said.

"We did well and were unlucky to concede at the end. But that's why they are champions, they were non-stop right until the last minute. It was hard to take.

"We just have to try not to let the result get us down because we did well. It won't be hard to bounce back because we can take so many positives from the game.

"We pushed the league leaders almost to the last minute with 10 men for half the game. It was some effort from the boys."

He is also hoping the clash with the champions will act as a springboard for himself in the SPL this season.

Offiong said: "I've had some niggling injuries here and there, which never help. But every player coming up from the First Division needs to adapt to the SPL, to the pace and getting to know the players.

"I think I've done that in recent weeks and hopefully I can kick on with a few goals now."