British American Tobacco (BAT) is preparing to raise about $25bn (£19bn) in the US and European bond markets this week to fund its takeover of American rival Reynolds.
The FTSE 100 tobacco giant will discuss the $49bn deal with global investors tomorrow.
The monster bond sale could run into difficulty, however, as the company faces a Serious Fraud Office investigation over bribery allegations in Africa, as well as a clampdown on nicotine products in America.
Banking sources said BAT may have to offer a higher interest rate on the debt to reward investors for the heightened risk.
The takeover is expected to close in the autumn, once competition watchdogs have reviewed it. BAT already owns 42% of Reynolds.