The British mercenary Simon Mann was today sentenced to 34 years in prison for plotting to overthrow the government of Equatorial Guinea.

The Eton-educated former SAS officer was sentenced after a trial last month during which it was claimed that a number of western governments knew about the coup plans. The court heard that Sir Mark Thatcher, the son of the former British prime minister, was a committed member of the group.

Mann was arrested in Harare, Zimbabwe, in 2004 with dozens of mercenaries when their private plane landed. He acknowledged knowingly taking part in the attempt to topple the government, but his lawyer argued Mann was a secondary player.

The sentence is longer than expected. During the trial, José Olo Obano, Equatorial Guinea's attorney general, urged the court to sentence Mann to 31 years, eight months and three days. The death penalty was not permitted under the terms of Mann's extradition from Zimbabwe. It was suggested last month that Mann may be released before completing any sentence.

Mann, wearing a grey prison uniform, stood impassively as the sentence was read out by the presiding judge, Carlos Mangue, in the heavily guarded courtroom.

Another defendant, Mohamed Salaam, a Lebanese businessman, received a jail sentence of 18 years, while four Equatorial Guinean nationals were given terms of six years each. Another was jailed for one year, and one other was acquitted.

Mann was ordered to pay a fine and compensation to the Equatorial Guinea state totalling around $24m (£12.1m). Mangue said in the ruling that Mann failed to show "an attitude of regret", despite his apology before the court.

During the trial in Malabo, a contrite Mann claimed he was "not the person I was" after four years in prison. He claimed that Spain and South Africa, with the endorsement of the South African president, Thabo Mbeki, had supported the plot. By January 2004, two months before the attempted coup was put into action, it was, Mann said, "like an official operation. The governments of Spain and South Africa were giving the green light: 'You've got to go, you've got to do it.'"

Senior members of the Equatorial Guinea army, police and cabinet were also implicated, Mann said, and he was given details of President Teodoro Obiang's daily movements and his health problems. From the Pentagon in Washington, and from the CIA and the big US oil companies, came tacit approval for regime change, according to Mann.

Thatcher "was not just an investor, he came completely on board and became a part of the management team", Mann claimed during his trial. He said Thatcher had provided $350,000 (£178,000) in funding for the coup.

Thatcher pleaded guilty in South Africa to unwittingly funding the purchase of an aircraft allegedly linked to the mercenaries. He was given a four-year suspended sentence and fined £265,000, after entering into a plea-bargain deal.

Mann was convicted in Zimbabwe of attempting to buy arms for an alleged coup plot and sentenced to seven years imprisonment. He was rendered from Zimbabwe to Equatorial Guinea in January. His lawyers accused Zimbabwean officials of a criminal conspiracy in secretly flying him out of the country before his appeals procedure was finished.

Teodoro Obiang Nguema Mbasogo has ruled the small west African nation of Equatorial Guinea since a 1979 coup overthrew his uncle. He pockets vast profits from offshore oil drilling. Equatorial Guinea is Africa's third-largest oil producer, but the bulk of its population remains very poor. Obiang's regime is accused of being one of Africa's worst violators of human rights.

Mann told the court that he took Thatcher to the Chelsea home of Ely Calil, the Lebanese businessman who is alleged by the government of Equatorial Guinea to have been the main financier of the plot. He named the management board as Calil, himself, a London property developer, Thatcher and a Lebanese colleague of Calil who lives in Beirut.

Calil has always strongly denied involvement in the coup plot and says Mann was pressured into repeated in court allegations that previously been extracted from him under torture in Zimbabwe.

Thatcher's money was used to buy a small plane that would transport the new provisional president, Severo Moto, from his opposition exile in Spain to Malabo via the Canary Islands.

Mann accepted he was doing the job for money – said to be $15m - but he claimed he was sympathetic to the story he was told that oil money was not reaching the people. "I believed it was right."