PARIS—A French court found former President Nicolas Sarkozy guilty of criminal conspiracy and sentenced him to five years in prison in a trial that examined whether his 2007 presidential campaign received millions of euros from the regime of Libyan strongman Moammar Gadhafi.

Under the ruling delivered by a panel of judges Thursday, the 70-year-old is required to begin serving his prison sentence within a month unless his lawyers can persuade judges to delay it while any appeal is being reviewed.

“If they absolutely want me to sleep in prison, I will sleep in prison, but with my head held high,” Sarkozy said, standing outside the courtroom next to his wife, Carla Bruni, who nodded in agreement. “I will fight until my last breath to prove my innocence,” he said, adding that he planned to appeal the ruling.

French President Nicolas Sarkozy (L) welcomes Libyan leader Muammar Gaddafi at Palais Elysee, Paris, France, 12 December 2007, (reissued 20 March 2018). EPA/MAYA VIDON

The presiding judge, Nathalie Gavarino, said Sarkozy was guilty of permitting his close associates “to act with the aim of obtaining financial support” from the Libyan regime. The judges, however, said there was no evidence that his 2007 campaign actually received Libyan funds. The judges acquitted Sarkozy on charges of concealing embezzlement, passive corruption and breaking campaign finance rules.

If incarcerated, Sarkozy would become the first former French president to serve jail time. Jacques Chirac , Sarkozy’s late predecessor, was convicted in 2011 of embezzling public funds and other charges, but his prison sentence was suspended. Sarkozy is currently serving a one-year sentence with an electronic bracelet for a separate influence-peddling case. Last year, a French appeals court also found him guilty of breaking campaign financing rules during his failed 2012 re-election bid. He has appealed that ruling.

Sarkozy’s prison sentence sent shockwaves through France’s political establishment. He remains an influential voice in the country’s politics with a strong following among French conservatives.

“I reiterate my support and gratitude for a statesman who gave so much to our country,” said Laurent Wauquiez , a lawmaker who leads Sarkozy’s conservative party in the National Assembly.

Far-right leader Marine Le Pen said the judges’ decision to enforce the prison sentence before the appeals process is exhausted “represented a great danger, given the main principles of our law.”

Prosecutors alleged that Sarkozy agreed to help rehabilitate Libya and Gadhafi on the world stage in exchange for funds to finance his presidential bid. Libya was considered a pariah state after the  1988 Pan Am Flight 103 bombing , an attack for which it admitted responsibility and which resulted in the deaths of 243 passengers and 16 crew members, as well as 11 others in the Scottish town of Lockerbie.

A few months after Sarkozy’s election in 2007, he welcomed Gadhafi to Paris with great pomp, allowing him to pitch his customary Bedouin-style tent across the street from the Élysée presidential palace. His visit caused outrage among the families of the Lockerbie bombing victims.

In his testimony, Sarkozy denied any allegations of wrongdoing, describing the accusations against him as grotesque and baseless. He said Gadhafi’s visit allowed France to sign contracts worth billions with Libya. Lawyers for Sarkozy argued in court there was no evidence of payments by the Libyan regime to Sarkozy’s campaign.

Sarkozy was ordered to stand trial along with 12 other men after an investigation that lasted more than a decade. Claude Guéant, who served as chief of staff to Sarkozy when he was interior minister, was found guilty of corruption on Thursday. Brice Hortefeux, a former minister and close Sarkozy aide, was convicted of criminal conspiracy.

The allegations against Sarkozy first came to public view in 2011, days before North Atlantic Treaty Organization countries launched strikes championed by him to support rebels fighting Gadhafi’s government. Gadhafi’s son, Saif, said in a television interview at the time that “Sarkozy must first give back the money he took from Libya to finance his electoral campaign.”

Sarkozy attributed Saif Gadhafi’s accusations at the time to a desire for revenge because of the leading role France played in the military operations in Libya.

The collapse of the Libyan regime complicated the French probe, officials involved in the case said. Documents were possibly lost or destroyed in the ensuing turmoil. Many potential witnesses were either killed or are living in countries beyond the reach of French authorities. Gadhafi was shot and killed in 2011 as Libyan forces overtook the city of Sirte, his last remaining stronghold.

In 2012, the body of former Libyan oil minister, Shukri Ghanem, was found floating in the Danube river in Vienna, the Austrian capital. Police later found a note in Ghanem’s 2007 diary, which meticulously documented his daily activities, and mentioned money transfers to Sarkozy totalling €6.5 million euros. Lawyers for Sarkozy questioned in court the authenticity of this diary.

In 2016, Ziad Takieddine, a French-Lebanese businessman, said he delivered suitcases full of Libyan cash to Sarkozy in 2006 to fund his campaign. In 2023, prosecutors pressed preliminary charges against Sarkozy for witness tampering and fraud, after Takieddine retracted his statement in a 2020 TV interview. Sarkozy told authorities that he had nothing to do with the interview, and that Takieddine’s statement lacked credibility due to his inconsistent accounts. Takieddine died on Tuesday in Beirut following a long illness.

Write to Noemie Bisserbe at noemie.bisserbe@wsj.com and Stacy Meichtry at Stacy.Meichtry@wsj.com