Sage Pay has apologised for an upgrade of its systems that left many of its 32,000 customers in the UK and Ireland unable to process transactions on Monday.

The problems, which rendered the payment processor's core system intermittently unavailable on Monday and continued to produce complaints on Tuesday, have been resolved, according to Mat Peck, head of R&D at Sage Pay.

The weekend upgrade was designed to make transaction processing faster and more resilient. Peck apologised for early teething problems that have accompanied the changes.

"As with any migration of this size, we've had lots of feedback, both positive and negative," Peck told El Reg.

"With regard to transaction processing, aside from some short-term, initial issues on Monday morning, transactions are being processed with no problems."

Complaints about the new interface have outnumbered gripes about Monday's transaction processing problems on the Sage Pay forums.

"Most of the feedback has been about the user interface (My Sage Pay)," Peck said. "We have a commitment to listen to, log and investigate this feedback and the forum you refer to is a vital tool in this process.

"We are investigating all queries raised by our customers, but I hope you'll appreciate that it will take time to do so in the manner our customers should expect of us." ®