Investor Warren Buffett is helping finance Burger King's planned takeover of Canadian coffee and doughnut chain Tim Hortons, according to people familiar with the matter, in a surprise twist that thrusts the billionaire into a debate over US taxes.

Buffett's Berkshire Hathaway would invest in the deal in the form of preferred shares, some of the people said. Berkshire is expected to provide about 25% of the deal's financing, one of the people said. The exact structure of Buffett's participation remains unclear and the discussions are ongoing.