In what should have been a moment of humiliation for any political party's leader, Van Schalkwyk declared: "I will personally encourage other NNP leaders and members to join the ANC.
"We will obviously have to do some convincing so those people can also join the ANC. But I must emphasise no one will be forced to join."
He said the essence of the decision was that it would be an individual choice.
Already rumbles of the party's death were audible after its meagre two percent poll in the last general election.
In the April poll the NNP returned to parliament with only seven members in the national assembly and two in the national council of provinces.
Van Schalkwyk said the recent adoption of the Freedom Charter by the NNP was a milestone in the party's quest to build a truly non-racial South Africa.
He announced the party's demise under the guise of a "new agreement" between the NNP and the ANC.
This means that the NNP will no longer contest forthcoming local government elections on its own but will ride on the back of the ANC.
The merger has to be approved by the ANC national executive committee, which is due to meet soon.
Van Schalkwyk said there could be people within his party who were not ready for a merger with the ANC.
To appease those reluctant Nats, Van Schalkwyk said, NNP leaders and members would be allowed to hold membership of the ANC and the NNP. Public representatives at council level could use a window period in September to cross the floor to join the ANC.
Aubrey Matshiqi, a political analyst based in Johannesburg, said the ANC had in any event been giving the NNP political mouth-to-mouth resuscitation.
"The historical irony is not lost on anyone. The NNP was the party of apartheid and the ANC was the main liberation movement against apartheid. Now the party of apartheid is being swallowed up by the party of liberation."
He said of the NNP's decision to cross over to the ANC: "It is death; Marthinus van Schalkwyk is hoping that, by not disbanding legally, the NNP will die a quiet death. In fact the Nats are dying an undignified death."
Mosiuoa Lekota, the ANC chairperson, who also attended the NNP's federal council meeting, did not gloat over his party's obvious triumph. Instead he said the two parties had been brought together by their commitment to non-racialism.
"If you look at the history of the ANC, you will see that we have always worked with other political organisations that shared our vision of non-racialism. Now that the Freedom Charter has become the foundation policy document of the NNP, we have decided to take our co-operation to another level," Lekota said.
The announcement came as no surprise to other politicians. Roelf Meyer, who left the NNP in 1997 because he found its refusal to change daunting, said the announcement "is almost a natural conclusion following their results in the last election.
"What is happening was predictable when I left. To my mind the National Party would have been able to stay on in a new format. In 1996 the National Party's withdrawal under FW de Klerk from the government of national unity was a terrible mistake."
Amichand Rajbansi, the leader of the Minority Front in KwaZulu-Natal, said the NNP would not be an asset to the ANC and could "hardly be expected to make a significant contribution to the organisation".
Bantu Holomisa, the leader of the United Democratic Front, welcomed the move and said it would strengthen relations between black and white voters who had previously voted on racial lines.
"It is good for a minority white party to collapse because, in essence, the future of political parties will be black, and this leaves a considerable challenge for the Democratic Alliance: to succeed where the Nats failed," he said.
Holomisa said he doubted whether die-hard NNP supporters would be angry as many had already deserted the party and found a home in the DA. "The move by Marthinus [Van Schalkwyk] should be welcome but my only concern is whether voters will trust parties when leaders switch allegiance shortly after elections," he said.
Helen Zille, the spokesperson for the DA, said the agreement was the final nail in the NNP's coffin and the last betrayal of their voters. "This has got nothing to do with political principle. Van Schalkwyk destroyed his party and abandoned his voters in return for a cabinet position. This agreement is the price he has to pay.
"The NNP voters will not follow him because they do not want to turn South Africa into a one-party state. The terrain is now very clear: it is the ANC versus the DA."
Retired politician Helen Suzman, always a fierce opponent of the Nats, said the announcement came as no surprise. "They [NNP leaders] are after a few more perks. They are going wherever they think they will get rewards and positions. They have no principles whatsoever," she said.
Professor Adam Habib, a political analyst at the University of KwaZulu-Natal, said the rationale of the NNP was increasingly difficult for the voter to understand, whether the voter was opposed to the status quo or for it. "The NNP took a drubbing at the polls because people perceived them to be completely opportunist. Already at a conference of its Free State branch earlier this year the NNP was debating whether it should continue existing."
Political analyst Tom Lodge said opposition would come from the right-wingers within the NNP - "but I wonder how many are left. They have so few MPs anyway. What is prompting this [the merger] is probably that the career politicians in the NNP want safe ANC seats."