"Inevitably the deal will be investigated, which could possibly take between nine and 12 months," Numis Securities Ltd. analyst Paul Richards told Britain's Independent newspaper.
Richards said the strength of the combined business in the United States is likely to be a concern.
Two years ago, the U.S. Justice Department investigated Reuters' acquisition of Telerate, a provider of real-time market data. That was a much smaller deal in terms of market impact, the Independent said.
"This deal would create an effective duopoly, with Thomson-Reuters and Bloomberg controlling over two-thirds of the market," Societe Generale's SG Securities analyst Anthony de Larrinaga said.
Regulators also likely will review if the deal would hurt smaller data providers, such as Dow Jones Newswires, which Reuters and Thomson distribute, the newspaper said.
Britain's National Union of Journalists has expressed concerns about job cuts following the deal. Thomson and Reuters have said the combined business would realize more than $495 million in savings.
Reuters employs 16,800 staff in 200 offices around the world, while Thomson's financial arm has 9,300 workers in 37 countries.