Mr de Klerk, South Africa's deputy President and former president, told a news conference that the move by his white-led, minority party "should be seen as an important step in the growing maturity ... of our young democracy".
He said South Africa needed a strong and confident opposition and added: "We intend to supply it." The National Party (NP) would leave mainly because a new constitution adopted on Wednesday made no provision for power-sharing after 1999. But he added: "It is not a crisis. We are not sour."
Mr de Klerk said his party would give up its seven cabinet seats in the 30-member government on 30 June, making sure Mandela had enough time to name successors for a smooth handover. The Zulu-based Inkatha Freedom Party is the third force in the government, though it is uncertain how the seats will be reallocated.
President Nelson Mandela said it would have been better if their partnership could have continued longer, but accepted the move as a "coming of age". But he warned: "The National Party has a continuing responsibility to contribute to the process of eradicating the legacy of apartheid which they created."
President Mandela said his policies would not change as they were premised on the needs of the people. Mr de Klerk's decision leaves the country wondering about the next move by the other minority party in the government, the Inkatha Freedom Party (IFN) led by Home Affairs minister Mangosuthu Buthelezi.
During his news conference Mr de Klerk said he was confident of South Africa's course. But away from the spotlight, there are clear signs that Mr de Klerk acted under pressure from his right wing, dissatisfied with a final constitution which failed to entrench state-funded exclusively Afrikaans schools.
For the NP's provincial leaders outside the Western Cape locked in a battle with General Constand Viljoen's Freedom Front for the soul of right wing white Afrikaaners, the schools issue was the last straw in a series of failures by the party to deliver to its core constituency.
The resurgence of the NP's right wing has temporarily put a spoke in the wheel of plans to realign the party as a broad church Volkspartei on the model of Germany's CDU.
Mr de Klerk became enamoured of the idea while on a visit to Germany last year, where discussions with Helmut Kohl and others convinced him that the time had arrived to move the NP away from its old ethnic and language focus to a more modern approach based on core values, including the market economy, Christian ethics, and a strong message of individual responsibility.
Things had looked so good in February, when Mr de Klerk's chief lieutenant, Roelf Meyer, resigned his cabinet post to become the full-time general secretary of the party. His job was to take the NP into the modern era, building alliances with Mr Buthelezi's IFP and the more liberal Democratic Party.
The strategy called for patience - Mr Meyer said again this week that it might take ten years before the new NP could even think of winning a majority this side of the ANC. But without it, the NP was - and is - condemned to the life expectancy of a dodo. The Freedom Front has more to say to ethnically minded white Afrikaaners, and the ANC's gains in last November's Western Cape elections showed that the NP's coloured vote is highly vulnerable. But as a party grouped around core values, the new NP would be free of its ethnic, language and historical baggage.
Building on Mr Kohl's model requires the creation of a centre-right party broad enough to integrate as many elements of the non-violent far-right as possible, while opening up to growing numbers of conservatively minded and increasingly middle class blacks.
But for the moment, the collective body language of the NP leadership at yesterday's news conference shows - white ministers and provincial leaders took centre-stage behind Mr de Klerk while black members of the group dallied on the fringes - that Mr de Klerk and Mr Meyer have an uphill battle ahead of them if they are to tame the right wing and get the party back on the Volkspartei track.
The ANC now has the greater freedom to manoeuvre in governing the country but it also will have to bear the responsibility for the results.Reuse content