Defending champion Angel Cabrera is alone in second place at 7-under-par 137 after a second-round 69. Jose Manuel Lara, who tied for second place last week at the Italian Open, shot a 4-under 68 to climb into a share of third place. Lara is tied for third with Jamie Elson (69) and Paul Casey (69) at 4-under-par 140.
Harrington started quickly with a birdie at the par-4 10th, his first hole at The Belfry. Two holes later, the Irishman dropped a shot with a bogey at the par-3 12th.
He atoned for that mistake with back-to-back birdies from the 14th. He continued his climb up the leaderboard with another birdie at the par-5 17th.
On the front side, Harrington eagled the par-5 third for the second straight day. He moved to 11-under with a birdie at the par-4 fifth. He had a chance to match the course record.
'It did cross my mind that I could equal that 64 and make sure of the course record when I eagled the third,' Harrington said. 'But to be honest there are not too many birdies chances when you get beyond the third.'
Harrington, much to his word, struggled to a double-bogey at the eighth hole, his 17th.
'My bad shots were more mental errors than anything else,' said Harrington. 'Then again, you have to expect to make some mistakes in a round of golf. That's the way it is.'
The mistake cut his lead from four shots to two heading to the weekend.
Cabrera also began his round on the back nine. He started with four consecutive pars. The Argentinean got going with four straight birdies from the 14th to climb to minus-8.
Around the turn, Cabrera managed just a 1-over 37 to drop back to two shots off the pace.
'I still like my chances,' Cabrera said. 'I played solidly on my front nine but not so good on the back nine. I don't feel too sure about my swing at the moment but I've been talking to my coach to try to sort it out.'
Emanuele Canonica, Julien Clement, Richard S. Johnson, Stephen Leaney, Rolf Muntz, Jean-Francois Remesy and Stephen Scahill are tied for sixth place 3-under-par 141.
Overnight leader David Dixon was unable to match his opening-round 66. He struggled to a 6-over 78 and is tied for 27th at even-par 144.
The cut line fell at 4-over-par 148. Among the notables not qualifying for the weekend are Ryder Cup vice captain Ian Woosnam (5-over 149), as well as former Ryder Cup players Darren Clarke (plus-8) and Nick Faldo (7-over).