ScottishPower will stop proactive sales for 12 days after failure to meet an Ofgem target to clear all outstanding Ombudsman decisions on customer complaints.
The ban started today (4 March 2015) and affects all outbound sales activities.
Ofgem launched an investigation into the firm in November of last year, after customers were experiencing long call waiting times and receiving late bills.
The energy supplier was also not implementing Ombudsman decisions.
ScottishPower signed up to three Ofgem targets to improve customer service within three months, or suspend proactive sales activities until the targets were met.
As the firm has failed to reach the target to remove a backlog for acting on Ombudsman decisions for individual complaints by the end of November 2014, they have now been hit by the sales ban.
Sarah Harrison, Ofgem’s senior partner in charge of enforcement, said that while Ofgem’s targets have driven "significant" improvements in ScottishPower’s performance, they remain "very concerned" about how customers are being treated.
She added: "As well as our ongoing investigation, we require ScottishPower to undertake an independent audit of its progress on improving customer service. We will keep the need for any further action under review.