British American Tobacco (BAT) is facing an investigation by the Serious Fraud Office (SFO) over claims of historic misconduct in Africa.
The allegations, first raised by a BBC panorama programme in 2015, involve BAT employees bribing politicians and civil servants in Rwanda, Burundi and the Comoros Islands to undermine anti-smoking laws.
Paul Hopkins, a former employee who worked for BAT in Kenya for 13 years, told the BBC that he had begun paying bribes to officials after being told it was the cost of doing business in Africa.
He said he would forward a tranche of documents to the SFO implicating BAT in misconduct.
The SFO confirmed on 1 August that it was "investigating suspicions of corruption in the conduct of business by BAT plc, its subsidiaries and associated persons".
BAT hired a team of external lawyers to investigate the allegations raised by Hopkins in February last year.
"As previously announced, we are investigating, through external legal advisers, allegations of misconduct," the British firm said in a statement.
"We have been cooperating with the Serious Fraud Office and British American Tobacco has been informed that the SFO has now opened a formal investigation. BAT intends to cooperate with that investigation."
Documents and emails shared by Hopkins revealed that BAT made illegal payments to two members and one former official of the World Health Organization's Framework Convention on Tobacco Control, which is aimed at reducing smoking-related deaths.
BAT is also alleged to have engaged in private-to-private corruption, with documents showing the company paid managers at a rival tobacco firm to gain a competitive advantage.
One of the UK's largest companies, BAT completed a $49.4bn (£37.4bn) buyout of American firm Reynolds to create the world's biggest tobacco firm in July.
Its share price fell initially but recovered to trade 1.5% higher in London following the SFO announcement.