Thomson Corp., owner of the Westlaw legal database and TradeWeb bond-trading network, plans to acquire Reuters Group Plc, the dominant service for currency trading, in an 8.77 billion pound ($17.5 billion) merger that would create the biggest financial news and information company.

Reuters shareholders will receive 697 pence per share, about half in cash and the rest in shares of Toronto-based Thomson, a Canadian newspaper company that transformed itself into a financial data supermarket. The offer is 13% more than Reuters’s closing price on May 4, when the negotiations were disclosed.

Thomson shares fell the most in 19 months on concern that the combined earnings of the two companies won’t justify the purchase price. At least three analysts downgraded their ratings on Reuters, partly because the new company, Thomson-Reuters, may be in violation of trade laws. London-based Reuters, the 156-year-old news organization founded by Paul Julius Reuter that has grown to 2,400 journalists in 131 countries, would triple Thomson’s share of the financial data market to 34%.

“There is a sense that Thomson is prepared to pay a fairly dear price for Reuters,” a manager of $9.6 billion at BMO Harris Private Banking in Toronto, Paul Taylor, said.

Analysts at Credit Suisse Group, Lehman Brothers Holdings Inc., and Deutsche Bank lowered their ratings on Reuters’s shares.