Romero hit a 67 while Ryder Cup hero Harrington had a 68 on the Old Course at St Andrews.

Harrington almost blotted his card, hitting a double bogey at the 17th to hand Romero, who had trailed by one stroke, the advantage.

But the Irishman stormed back with a birdie at the last to join the Argentine in pole position to claim the �500,000 first prize.

Harrington had begun well, making birdies at the 1st and 3rd holes to move to 14 under par, three clear of Romero.

Romero, the Scottish Open champion, could only par the first two.

But he soon found his rhythm to apply the pressure on Harrington at the top of the leaderboard as he picked up three shots to move to 14 under.

Harrington stayed one ahead after he made another birdie and the pair traded birdies as they remained locked at the top of the leaderboard.

Harrington, who is looking for his first win of the season, said his double bogey had taken the gloss off his round.

"I suppose 68 is a decent score but I had been thinking of a 66 to put a bit of space between me and the guys behind," he said.

Among the pack three shots back is Vijay Singh, who had a brilliant 64, which included an eagle at the 18th.

Sandy Lyle also stayed in the hunt with a five under par round.

"I'm playing a lot better this week bit I'm obviously apprehensive because I haven't had a really good week for a long time," said the Scot.

South Africans Retief Goosen and Ernie Els are well clear on the money list but third-placed Harrington could move right up on their heels if he wins the title.

Els was playing himself into contention for this title but his chances were dashed with a quadruple bogey at the 17th.