Mishcon de Reya's London office

Mishcon de Reya is being sued for breach of contract and negligence by a former client who lost a £15 million court battle against retail mogul Mike Ashley.

Banker Jeffrey Blue is suing the firm, claiming he could go bankrupt as a result of the case's high costs, according to legal documents filed at the High Court in London. Blue has claimed that Mishcon failed to "obtain adequate litigation finance or after-the-event insurance, as a result of which he suffered significant loss".

The original claim document referred to Blue as a consultant with Ashley's retail chain, Sports Direct, which he joined in 2012. Blue claims that, during a drinking session in 2013, Ashley agreed to pay him £15 million if the share price of Sports Direct reached £8 per share within three years, which it did, though Blue did not receive the sum from Ashley.

Among several accusations against the firm, Blue claims Mishcon had "failed to obtain adequate litigation funding" and "failed to properly supervise" one of its managing associates working on the case who obtained the funding and insurance, which Blue described as inadequate.

The case was dismissed, with the judge saying: "The fact that Mr Blue has since convinced himself that the offer [by Mr Ashley during banter in a pub] was a serious one, and that a legally binding agreement was made, shows only that the human capacity for wishful thinking knows few bounds."

In October last year, Blue was ordered to pay Ashley's legal costs of nearly £1.5 million on top of his own legal costs, according to the documents.

Mishcon disputes Blue's latest claim, calling it "the product of hindsight and wishful thinking", according to the filing.

Mishcon did not respond to requests for comment. A legal representative for Blue declined to comment.