Bankers unhappy with their bonus get set to sue

Disgruntled bankers disappointed with their Christmas bonuses could fuel a surge in legal cases bought against their employers, a leading City law firm has warned.

The number of High Court cases relating to "breach of contract" – the majority of which are related to bonuses -- jumped from 456 in 2010 to 809 last year, the highest number on record, Nockolds research showed, and this is set to rise.

The law firm said disappointing bonuses may force City workers to sue their workplace, but one of the biggest drivers could be confusion over contracts that allow verbal promises of a bonus to be legally binding.

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"In recent years there have been a number of cases whereby verbal promises have led to expectations and so (in a) major case…(the)court of appeal said, 'look if employers make verbal promises to employees then that's considered to be a contractual variation and a legally binding contract'," Darren Hayward, head of employment at Nockolds, told CNBC in a TV interview.

In May 2012, Commerzbank lost a landmark High Court case brought by a group of bankers who were promised a bonus pool of 400 million euros ($491 million) in 2008.

Banks under fire

Data from salary benchmarking website Emolument earlier this year showed bonuses in some City jobs are set for dramatic falls. Currency traders could see a 42.9 percent drop in bonuses during the next payout season in 2015, while emerging market and equity derivative traders could see bonuses slashed by 30 percent.

Bankers' bonuses have been in the spotlight this year as European regulators have looked to clamp down on what they view as large rewards that fuel excessive risk taking. The European Union unveiled rules last year that limit bonuses to no more than a banker's salary, or double that level with shareholder approval.

Banks have also been under fire from financial regulators who slapped five global banks with $3.4 billion worth of fines over the rigging of key foreign exchange markets.

Read MoreFX bonuses set to plunge on probes, low volumes

Against this backdrop, Hayward said banks might be reluctant to pay out the bigger bonuses that employees are expecting.

"I think banks are conscious about public perception of bonus levels. I also think they need to improve their balance sheets and profitability," Hayward said.

But some lawyers disagree with Nockolds' forecast of a rise in legal challenges this Christmas, suggesting that cases like Commerzbank in 2012 are unusual.

"While some employees may be unhappy if they experience a lean year, over the years the courts have made it clear that that the key test – to demonstrate that the award was irrational – requires a very high threshold," Monica Kurnatowska, employment partner at Baker & McKenzie, told CNBC by phone.