Belgium's former king has admitted he fathered a love child during an extramarital affair in the late 1960s.

Albert II was found to be the biological father of 51-year-old Belgian artist Delphine Boël after a court told him to take a DNA test.

Lawyers for the 85-year-old said in a statement printed in RTBF that he acknowledged his biological connection, but not his "social" role as a parent.

"King Albert wishes to point out that, since the birth of Madame Delphine Boël, he was not involved in any family, social or educational decision whatsoever relating to Madame Delphine Boël," it read.

The statement went on to say Albert — who abdicated in 2013 due to poor health — had "respected the link that existed between Madame Delphine Boël and her legal father."

Former king Albert (right) has said he had his wife Paola (left) survived a 'crisis' in the late 1960s 2008 AP Virginia Mayo

The admission comes after seven years of court battles against Boël's paternity claim and requests for him to undergo a DNA test.

But rumours had begun circulating much earlier about Albert's affair with Boël's mother, Baroness Sybille de Selys Longchamps, following the 1999 publication of a biography about his wife, Queen Paola.

In his Christmas message to the nation that year, Albert alluded to past infidelity by saying he and Paola had lived through a "crisis" in the late 1960s that almost wrecked their marriage.

He went on to assure it was a "long while ago" and that the couple had overcome their marital problems.

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His acknowledgement of his fourth child on Tuesday was met with "relief and emotion", but also "a great pain" from Boël, whose lawyer said it was because "she lacked a father".

The statement added: "Recognition was to prevent her children from experiencing the same.

"The words of [Albert's] lawyer are stronger than the press release, which is not very elegant because he poses as a victim.

"There was an outstretched hand from Delphine Boël to communicate on these results, but the press release is icy and does not reflect any form of affection. "