Burger King in takeover talks with Tim Hortons
Burger King has said it is in takeover talks with Tim Hortons, the Canadian coffee and doughnut chain.
A merger would create the world's third-largest fast-food combine, one with a stock market value of about $18bn (£10.9bn; 13.6bn euros).
At close of the US stock market on Monday, Burger King's shares were up by 19.5%, and those in Tim Hortons by 19%.
The firms have said that any new group would have its HQ in Canada, where corporate taxes are lower.
These so-called "tax inversion" deals are attracting increasing criticism in the US, where President Barack Obama is understood to be looking at how they can be prevented in future.
The US corporate tax rate is 35%, but 26.5% in Ontario, Canada, where Tim Hortons is based.
Burger King's majority shareholder, 3G Capital, would stay in overall control.
The New York and Rio de Janeiro-based investment company bought Burger King in 2010 for about $3.3bn and floated the company in 2012, holding on to nearly 70% of the shares.
If a deal goes ahead, the remaining shares will be distributed between the current shareholders of Burger King and Tim Hortons.
According to reports, the companies will retain their separate brand identities but save costs by sharing corporate services.
Combined, Burger King and Tim Hortons would have an estimated revenue of $22bn a year from around 18,000 restaurants in 100 countries.
Tim Hortons used to be owned by US fast-food chain Wendy's, before being spun off as a separate company in 2006.