Former apartheid president FW de Klerk has cut short his Europe trip because of former president Nelson's Mandela's ill-health, De Klerk's foundation said on its website.

"Mr FW de Klerk and his wife Elita have decided to suspend their current working visit and holiday in Europe because of Mr Mandela's medical condition," it said.

"They will arrive back in Cape Town tomorrow. Their thoughts are with the Mandela family during this difficult time and they join in the prayers for an improvement in Mr Mandela's health."

There was no further information on whether De Klerk would be visiting Mandela.

It has been three weeks since Mandela was admitted to the Medi-Clinic Heart Hospital in Pretoria.

On Saturday, President Jacob Zuma said his condition was still critical but stable.

"We're hoping that it [Mandela's health] is going to improve," Zuma said at a press briefing on Saturday.

"I think with all the prayers and good wishes that are being made, certainly it would do something."