South Korea’s Park Inbee celebrated her 26th birthday by grabbing a one-shot lead at the Women’s British Open after compatriot Ahn Sun Ju was penalised two strokes.

With a third round four under par 68, the former world number one claimed the lead after a dramatic end to the third day at Royal Birkdale.

Ahn thought she had taken the lead on five under par after a 69.

But she was then penalised two shots for infringing Rule 13-3/3 — in a greenside bunker at the final hole, she was deemed to have “built a stance” and so was docked the two shots for a 71.

It put her on three under par and in a share of second place with Norway’s Suzann Pettersen and China’s Feng Shanshan.

Pettersen, a two-time major winner, had a 68, while Feng, the 2012 LPGA Champion, shot 69.

Ahn, with 26 wins in Korea and Japan, admitted it had been pure ignorance which had caused the penalty.

“I didn’t know about the rule but all I was doing was trying to make a stance,” said the 26-year-old.

“I am surprised by it but if that’s the rule then I have to abide by it and just refocus. The ball was placed on a very upslope lie so it was hard to make a stance.

“My intent wasn’t to break the rule, just to set my feet firm. I’ll have to have a night’s sleep and then see if it is going to motivate me.”

Twelve months ago, Park, now the world number three, was trying to make history by sweeping a fourth season’s major before coming up short.

She started Saturday with back-to-back birdies, made another at the fourth and yet another at the sixth. The first three were from 15 feet, the fourth from half that distance.

“I hit every green and every fairway today,” she said.

“The first four holes are so hard but I was three under and so I couldn’t have asked for a better start. Compared to last year I do feel much more mature.”

Stacy Lewis, the American who ended Park’s four in a row attempt in 2013 at St Andrews, kept up her hopes of a successful defence with two birdies in the final three holes for a 70 and one under par.

The round of the day and the championship was a six-under-par 66 from English 18-year-old, Charley Hull.

Throwing caution to the wind, the teenager had nine birdies and two at all the four short holes and was alongside Lewis, three shots off the lead.