U.K. companies' legal liabilities soared 12 percent in 2013, driven by the hefty fines slapped on the banking sector, according to Thomson Reuters' legal business.

FTSE 100-listed companies set aside £24.6 billion ($40.5 billion) in 2013 to deal with complaints and lawsuits, up from £22.1 billion in 2012. "There is a perception that regulators and government have responded to public pressure to show no mercy for large corporates that have been caught breaching laws and regulations," Raichel Hopkinson, head of dispute resolution at Thomson Reuters, said in a news release. "Businesses hope that as the U.K. economy grows and memory of the recession fades that they will face lower levels of legal risk but that hasn't happened yet," she added.

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Oil and gas sector takes largest hit The energy sector also suffered from rising liabilities in 2013, with firms setting aside £9.9 billion to cover litigation, up 22 percent from £8.1 billion in 2012. Thomson Reuters said over half of the value of oil and gas litigation provisions were in response to the 2010 Gulf of Mexico oil spill. (Read more: JPMorgan pays hefty cost for 'Whale', settles for $920 million)

"The risk of claims arising out of new developments in shale plays in the U.K., and the shifting regulatory framework, mean that higher legal provisions in the oil and gas sector are unlikely to subside, despite an expectation that legal costs would fall when BP finalizes its Gulf of Mexico settlement," Hopkinson added. Earlier this month, BP's appeal to curb the multi-million dollar pay-outs for claims relating to the 2010 oil disaster were thrown out by a U.S. court.