Spain's disgraced former king Juan Carlos is being sued for alleged tax offences by a group of retired judges, one of the complainants said on Monday.

Spanish prosecutors in March 2022 closed three probes into his finances due to insufficient evidence and the statute of limitations, but said several irregularities had been found.

Among them were lucrative gifts from Gulf Arab monarchies and private trips funded by a foundation based in the tiny European state of Liechtenstein.

The complainants say a court rather than public prosecutors must decide if Juan Carlos fulfilled his tax obligations.

Juan Carlos, 86, has made two payments worth more than five million euros to the Spanish tax authorities to settle money owed on undeclared income, widely viewed as a bid to avoid charges.

His regularisation of income declarations from 2014 to 2018 "were not done in accordance with the law", former Spanish Supreme Court judge Jose Antonio Martin Pallin told public broadcaster TVE.

"We do not have the slightest interest in prison sentences being applied," but the harshest possible fine should be imposed, he added.

Legal experts, philosophers and journalists were also among the complainants, Pallin said, without specifying their number.

Juan Carlos enjoyed widespread admiration for his role in Spain's transition to democracy following the death of longtime dictator Francisco Franco in 1975.

But the revelations about the murky origins of his fortune since his abdication in 2014 have done irreparable damage to his reputation and harmed the Spanish monarchy.

He went into self-imposed exile in the United Arab Emirates in 2020. (AFP)