Cotonou, Benin - A bodyguard for Benin President Thomas Boni Yayi has been detained and questioned over an alleged plot to poison his boss which has already led to three arrests, the public prosecutor said on Wednesday.

“The president's bodyguard was actually questioned and detained over the poisoning saga,” public prosecutor Justin Gbenameto told reporters.

Bachirou Adjani becomes the fourth person to be detained over the mysterious alleged plot to poison the Benin president. The three others, arrested last month, were also close to Yayi.

A former trade minister, the president's niece and his doctor have been charged over an alleged conspiracy to replace Yayi's anti-pain medicine with poison in a case that has rocked the small West African country.

Former trade minister Mudjaidou Soumanou was arrested on October 21 and charged along with Yayi's doctor Ibrahim Mama Cisse and niece Zouberath Kora-Seke with criminal conspiracy and attempted murder.

Two others are wanted in the case, including the alleged mastermind, businessman Patrice Talon, and his associate Olivier Boko. Talon is a former ally of the president but the two have since fallen out.

Asked why Ajani was questioned, a close advisor of Yayi, who demanded anonymity, said it was due to recent “closeness” between the bodyguard and Talon.

There has been speculation that the alleged plot may have been linked to lucrative contracts involving the port and the cotton industry.

The allegations have raised some suspicions in Benin that they are intended to smear the president's enemies, and members of the opposition have criticised the case.

Yayi, 60, is an economist who first took office in 2006 and won re-election last year with 53 percent of the vote. - Sapa-AFP