The group action follows West Brom’s decision to increase buy-to-let tracker rates by two percentage points last year. The landlord campaign group said the lender had no legal grounds to do so.

The group had initially sought to represent 250 cases but by the deadline 401 affected mortgage accounts had signed up to the action, raising funds of £458,744.

Mark Alexander, leader of the campaign against the West Brom and an affected borrower, said despite the deadline there was still an opportunity for more to join the action.

“It is theoretically still possible for people to join the action right up until the date of the court hearing being announced,” he said. “However, this will require additional administration and for fees to be paid to the courts to add details of latecomers to the representative action.

“To cover these costs a premium of £250 will be levied. But it would be churlish not to accept latecomers given that their contributions reduce the financial risks to all members.”

Cotswold Barristers has been instructed by campaign members to bring the claim to court and any borrowers wishing to add their name to the action should contact the firm immediately.