A former warlord and ex-vice-president of the Democratic Republic of the Congo has been sentenced by the international criminal court to a year in jail and fined €300,000 (£260,000) for bribing witnesses during an earlier war crimes trial at the ICC.

Jean-Pierre Bemba is already serving an 18-year sentence for war crimes committed by his marauding troops, which he sent into Central African Republic in 2002-03 to put down a coup against the then president.

Wednesday’s verdict and sentence are the first of their kind in the history of the ICC. Bemba, 54, was convicted of masterminding a network to bribe and manipulate at least 14 key witnesses, and had “planned, authorised, and approved the illicit coaching” of the witnesses to get them to lie at his main trial.

The presiding judge, Bertram Schmitt, told Bemba the “substantial fine” was necessary “to discourage this kind of behaviour”.

The year-long sentence will run consecutively to his 18 years’ jail time.

Members of Bemba’s legal team also received jail terms. Aime Kilolo, his lawyer, received the heaviest sentence among four of the former vice-president’s associates: two years and six months for “abuse of trust” as well as “abuse of the lawyer-client privilege”. Kilolo, who was arrested in 2013, was also ordered to pay a €30,000 fine.

Set up in 2002 to prosecute the world’s worst crimes, the ICC makes significant – if not always successful – efforts to try to protect witnesses and its trials from any interference.

In 2015, the ICC was forced to drop charges against Kenya’s president Uhuru Kenyatta, who had been accused of stoking ethnic violence after Kenya’s 2007 presidential election.

Prosecutors blamed their failure to put Kenyatta on trial on political interference and massive interference with witnesses, especially after Kenyatta was elected president in 2013. In April, charges against Kenyatta’s deputy, William Ruto, were also dropped for similar reasons.

“The type of sentence, whether heavy or light, will send a clear message about the gravity of the crime,” said Mariana Pena, from the Open Society Justice Initiative, an international law advocacy group.

Bemba became the third person convicted by the controversial “court of last resort” and his main trial was the first before the ICC to focus on sexual violence as a weapon of war, and the first to find a military commander to blame for the atrocities perpetrated by forces even though he did not order them.

A series of rapes, murders and atrocities were committed in CAR between October 2002 and March 2003 by troops from Bemba’s Congolese Liberation Movement (MLC). The judges said Bemba could at any point have ended his militia’s five-month rampage, but chose not to.

A wealthy businessman who became a key player in the chaotic civil war in Congo between 1998 and 2002, Bemba rose to be one of four vice-presidents in the transitional government of DRC’s president, Joseph Kabila. In 2006, he lost to Kabila in a presidential election runoff and fled to Europe. He was arrested in 2008 in Brussels and handed over to the ICC.

His MLC militia has since morphed into a political party and is currently the second-largest opposition group in the national assembly.

Some analysts have said the conviction of Bemba merely highlighted the failure of ICC to punish anyone for the widespread and systematic human rights abuses committed by militia in DRC itself.

The vast central African country remains mired in violence and political instability. Kabila has stayed in power despite his electoral mandate expiring in December last year. The government and a fragmented opposition have failed to come to a deal despite months of negotiations, though there remain hopes new polls may be held later this year.

There is also rising unrest in the centre and east of DRC, raising fears of regional intervention and a possible return to civil war.

The ICC has repeatedly faced criticism by some in Africa who regard it as racist or imperialist. Burundi, South Africa and the Gambia have announced plans to leave the court, leading to concerns other states would follow.

In a boost for the ICC, however, Adama Barrow, the new democratically elected president of the Gambia, recently reversed that decision – made by Yahya Jammeh, the west African state’s former authoritarian leader – while the South African government was forced to abandon its notice of withdrawal after a judge found it unconstitutional.