The family of a Belgian aid worker seized in Iran have revealed the grim conditions he is being held in as they plead for his release.

Olivier Vandecasteele, 41, was arrested in the hardline Islamic state in February, accused of spying.

He is now being held in the notorious Evin Prison in the capital, Tehran, where the rape and torture of inmates are said to be common.

Critics say he is being held on bogus allegations to use him as a bargaining chip for an Iranian diplomat imprisoned on terror charges in Belgium.

Olivier’s sister, Nathalie Vandercasteele, said: “Even though he’s innocent, he has been rotting away for nearly five months in total solitary confinement.”

She added that her brother had lost a lot of weight and had developed a foot infection.

She said these were discovered during the two consular visits he received.

Nathalie revealed: “He has spent two months without even a mattress, in a cell lit up around the clock, and being subjected to daily psychological pressure from interrogators.”

She has accused Belgium’s authorities of not doing all they can to secure her brother’s release.

She said: “It is unthinkable for our family that our democratic Belgium isn’t doing everything necessary to get innocent prisoners out of countries like Iran.”

Belgian representatives and rights groups believe Iran jailed Olivier to force Brussels into releasing the Iranian diplomat Assadollah Assadi, 50.

Assadi was jailed in February last year for “attempted murder and involvement in terrorism” after he was determined to have been involved in an attempted bombing at a gathering of the National Council of Resistance of Iran – a political organisation opposing the Iranian regime – in Villepinte, France.

Tehran rejects this finding.

The Belgian government is now urging the country’s Federal Parliament to pass a contentious prisoner-exchange agreement with Iran that could see Olivier and Assadi swapped.