Belgian Foreign Minister Sophie Wilmès says she will temporarily step down to care for her husband, who has been diagnosed with brain cancer.

Ms Wilmès, who also serves as deputy prime minister, will take leave until "the end of the summer", when her family will reassess the situation.

She says she made the move "in perfect consultation with the prime minister".

In a statement, Prime Minister Alexander De Croo said Ms Wilmès's choice "commands the greatest respect".

Mr De Croo added that the minister would not officially resign from government and that her powers would, instead, be temporarily transferred to other ministers by "a royal decree".

Ms Wilmès, who served as prime minister between 2019 and 2020, married her Australian husband Chris Stone in 2009.

A former Australian Rules footballer with St Kilda, he has been the vice-president of the Belgian branch of Australian Business in Europe since 2012.

The two have three daughters. Mr Stone also has a son from a previous relationship.

"The disease has suddenly entered our lives and, in particular, that of my husband, Christopher, who must begin, like far too many men, women and even children, his fight against aggressive cancer of the brain," Ms Wilmès wrote in a statement posted to Twitter.

"Being a minister requires rigour, availability and a total commitment which would not allow me to provide the help and comfort that Christopher and our children will need during this difficult period," she went on.

Mr De Croo said that he would temporarily assume Ms Wilmès's position as foreign minister.

"In an ordeal like this, it is the spouse, the mother who comes first. I wish Sophie, Christopher and their children a lot of courage and strength," Mr De Croo said.