British American Tobacco (BAT), one of Britain's biggest companies, was involved in a bribery negotiation with the former Zimbabwean leader Robert Mugabe's Zanu-PF party in 2013.
According to a BBC Panorama investigation, the company paid a bribe between US$300,000 and US$500,000 to Mugabe's political party.
The BBC documents also reveal BAT was paying bribes in South Africa and using illegal surveillance to damage rivals.
While President Mugabe's 37-year rule was secured through elections marred by allegations of fraud and violence, BAT in its defense said that it is committed to the highest standards of corporate conduct.
In a joint investigation with the Bureau of Investigative Journalism and the University of Bath, Panorama obtained thousands of leaked documents that show how BAT funded a network of almost 200 secret informants in southern Africa.
Most of this work was outsourced to a South African private security company called Forensic Security Services (FSS).
FSS was officially tasked with fighting the black-market cigarette trade, however former employees have told the BBC that they broke the law to sabotage BAT's rivals.
Internal documents show in one operation, FSS staff were instructed to close down three cigarette factories run by BAT's competitors in Zimbabwe.
FSS paid a local firm to conduct surveillance on a Savanna Tobacco factory in 2012, but the company got caught.
Three of its directors were charged in connection with illegal surveillance. The arrests prompted the then president, Robert Mugabe, to make a speech condemning the men's actions and BAT's suspected involvement.
However, Panorama has found that behind the scenes, contractors working on behalf of BAT were talking to Zimbabwean officials.
The man who was sent in to negotiate a deal, who wishes to remain anonymous, told Panorama he bribed a number of government officials to secure a meeting to discuss the men's case.
He said: "I had to make it clear that they're going to expect a nice thick envelope of notes."
Documents seen by the BBC confirm that the man was provided with the equivalent of US$12,000 in local currency. They also suggest the money for the bribes was provided by BAT.
An internal memo outlines the deal that was proposed. The Zimbabwean official said that with the upcoming presidential elections, a donation to Mugabe's party - Zanu PF - would help.
The memo said: "With this donation, they could then go back to the President" to try to get the problem sorted out.
"The amount of the donation would have to be in the region of between USD300,000 to USD500,000 to Zanu PF," it said.
The documents do not show whether the bribe was in the end paid, but Panorama has spoken to three sources who have confirmed that BAT was aware of the terms of the deal on offer.
Within days of the deal being offered, all three directors were free.
BAT declined to answer Panorama's questions about the Zimbabwe payments, but it did not deny paying a bribe to Robert Mugabe.
It is against UK law for a British company to pay bribes, no matter where the payment takes place.
FSS worked for BAT in southern Africa between 2000 and 2016.
Evidence strongly suggests they bribed customs officials and police officers, and that BAT secured access to information from the police camera network, which was used to spy on its rivals.
FSS tapped the phones of BAT's competitors, placed tracking devices on their delivery vehicles and bribed staff to hand over information.
Documents show senior staff from BAT's London HQ personally recruited and paid some of the informants working at competitors' factories.
BAT staff would also load cash onto currency cards in London and the informants could then withdraw the money anonymously in South Africa.
It is a payment system sometimes used by organised crime groups to evade detection.
BAT's lawyers said the allegations are not new and that it was not unlawful to pay sources to gather information about criminal behaviour.
They said the company rejects the allegation that any steps were taken with the aim of impacting the lawful activities of legitimate competitors or for commercial advantage.
BAT said: "We emphatically reject the mischaracterisation of our conduct... Our efforts in combating illicit trade have been aimed at helping law enforcement agencies in the fight against the criminal trade in tobacco products.
"Acting responsibly and with integrity underpins the foundations of our culture."
This is not the first time BAT has been accused of bribery. In 2015, Panorama found the company had secretly paid politicians and civil servants in countries in east Africa to undermine anti-smoking measures.
The revelations prompted a five-year investigation by the UK's Serious Fraud Office (SFO). Earlier this year, it concluded there was not enough evidence for prosecution.
BAT says it fully cooperated with the SFO's investigation, which included allegations relating to South Africa and which resulted in no action being taken.