Law firm Leigh Day are investigating the potential for legal claims following the decision by the Court of Appeal that The West Bromwich Mortgage Company should refund £27.5m to thousands of home-owners, mostly landlords, who say they were overcharged.

The West Bromwich Mortgage Company, an arm of West Bromwich Building Society, was found to have acted unlawfully when it increased its charges for buy-to-let tracker mortgages.

Even though base rates had not risen, the lender decided to increase in its tracker rate to 1.9% in September 2013, Tracker mortgages are advertised as following the Bank of England's base rate, which has remained at 0.5% since 2009.

According to reports as many as 6,500 landlords, with multiple properties, could now be refunded an average sum of approximately £4000 each.

The initial case was brought by Mark Alexander, however, the High Court ruled against him, but the Court of Appeal has now reversed that decision.

The West Bromwich Building Society said it would record a loss in the current financial year as a result, but that its capital position remained strong.

Jonathan Westhoff, the chief executive of The West Bromwich Building Society said: "At all times we acted to ensure we were treating customers fairly and that our approach was in the best interests of the Society and its members as a whole."

Michelle Victor a partner in the Consumer Law and Product Safety group at Leigh Day said: “We would urge anyone who believes they were affected by the decision of The West Bromwich Mortgage Company to come forward as we investigate legal action against the company.

“We are currently investigating whether to pursue a group claim against the organisation to recover any direct losses incurred by home owners following the decision of the building society to increase its rates, as well as other costs incurred through this unlawful action.”