Payday lender Wonga is to pay more than £2.6 million in compensation to around 45,000 customers for “unfair and misleading debt collection practices”, the UK markets regulator has announced.

The UK’s biggest payday lender was found to have sent letters to customers in arrears from non-existent law firms threatening legal action, the Financial Conduct Authority (FCA) said.

In some cases, Wonga added charges to customers’ accounts to cover administration fees for sending the letters.

Wonga apologised “unreservedly” for the failings, which took place between October 2008 and November 2010.

The FCA said consumers were put under “great pressure” from communications sent by fictitious law firms to make loan repayments that many could not afford.

Wonga contacted customers in arrears under the names Chainey, D’Amato & Shannon and Barker and Lowe Legal Recoveries, leading customers to believe that their outstanding debt had been passed to a law firm, or other third party. Further legal action was threatened if the debt was not repaid.

Neither of these firms existed and Wonga was using this tactic to maximise collections by piling the pressure on customers, the regulator said.