Denmark's Soren Hansen fired a flawless 8-under 64 - the best round this week at the Marriott Forest of Arden -to rocket into a share of second place at 12-under with Steve Webster of England, who was also bogey-free in shooting a third-round 65.
Englishmen Sam Walker (66) and Justin Rose (68), the winner of last week's British Masters, finished together at 11-under 205, while Phillip Price (70) of Wales was alone in seventh at minus 10.
Clarke, seeking to win this tournament for the third time in four years, tallied a pair of birdies on the front nine Saturday to get to 11-under. After posting four consecutive pars around the turn Clarke knocked a 5-iron to within four feet at the par-5 12th to set up an easy eagle.
He rolled in another four-footer at the 13th for a birdie that gave him the outright lead at 14-under par. But he dropped a shot at the par-5 17th, where he missed the fairway then the green en route to a bogey six.
'That was a little bit disappointing because the 17th was the only poor tee shot I had hit all day,' said Clarke, who hit 12 of 14 fairways for the third day in a row.
'I have been hitting the ball well for some time now but haven't been able to turn it into the scores. I am now starting to do that, though. Sometimes itcl and running.'
The 33-year-old Clarke has five top-five finishes in six English Open starts at the Forest of Arden course, including a win in the 2000 event. He captured his first English Open title the year before at Hanbury Manor.
Jacquelin, whose runner-up showing here last year marked his best performance on the European Tour, experienced a roller-coaster third round - most of it up - that produced a 7-under 65, his lowest score on tour this season.
He started with five birdies through the first seven holes before collecting two birdies, two bogeys and two pars over the next six. The 28-year-old Frenchman picked up shots on 15, 16 and 18 but mixed a bogey at the 17th.
'It was a very strange day because I didn't drive as well as I did on Thursday or Friday,' said Jacquelin, who found the fairway off the tee just eight of 14 times in round three. 'I missed a few fairways but the putter was very hot. I think I made 24 putts which is pretty good putting statistics for me.
'It was a bit of fun but that's what we all need out there.'
Soren Hansen is in the hunt for his breakthrough win, two weeks after his countryman and namesake Anders Hansen won his first title at the Volvo PGA Championship.
'I'm on the right path,' Soren said. 'It is just a matter of time really and I think the more I'm there the sooner it will happen, and then I think it will happen on a regular basis.'
He made six birdies and added an eagle at the 12th, where he pitched in from 15 feet off the green. He took only 23 putts on Saturday.
European Ryder Cup captain Sam Torrance shot a 65, his best round since the 2000 Italian Open. The 48-year-old Scot had a chance to post a 64 but took a bogey at the final hole. He finished four shots off the pace with four others, among them two-time major winner Sandy Lyle.
A total of 22 players stood within six shots of the lead with one round to play.
Clarke's second-round co-leader David Drysdale of Scotland was five strokes back at 8-under. Australian Jarrod Moseley, who shared the opening-round lead with the Ulsterman, slipped back to minus 4.
Full-field scores from The Compass Group English Open