By Duncan Guy
Tony Leon will step down as leader of the Democratic Alliance in May, he told reporters in Johannesburg on Sunday.
He said 13 years in office was "the absolute upper limit of effective leadership".
"There is a danger, over time, that no matter how healthy or vigorous the internal workings of an organisation... that the identity and branding of the party will be almost completely absorbed into the identity and personality of its leader," Leon said.
"This is not good for the health of the party - or the nation - and come to think of it, not particularly good for my health either."
Leon said he would remain a member of parliament and continue writing a book he has been commissioned to publish.
He emphasised that he had no desire to influence the choice of his successor, and wanted the DA to have good time to prepare for the election of its next leader.
"I wish to make it clear that I do not belong to this dismal school of political leadership. Our party must freely and democratically choose its leadership, without let or hindrance."
He added that this was part of trusting one's organisation.
The party will hold its federal congress in May next year.
Leon said: "In the light of my commitment to the party and its internal stability it was my intention to announce my decision closer to the congress in May 2007.
"However, speculation has already arisen - prematurely and publicly - in a different quarter.
"I have no wish, nor does our party, and its millions of supporters deserve to be involved in debate... on either my merits or an argument regarding continuance in office."
Leon said he made the decision four months ago.
He said the DA was almost unrecognisable from the "shattered organisation" he was bequeathed after the 1994 election.
Asked about its appeal to black voters, Leon said the DA had more black support than two "black" parties with seats in parliament - the Azanian People's Organisation (Azapo) and the Pan Africanist Congress of South Africa (PAC).
"South Africa needs to move to the point where people will vote on issues, not identity.
"We are not at that point. We must and will make sure we make a contribution to that goal."
Leon singled out his involvement in helping to write the constitution in 1993 and 1996 as a high point of his career.
"I enjoyed it. I now look at certain clauses and I know exactly how they got there."
The Congress of South African Trade Unions (Cosatu) welcomed Leon's announcement, saying he would do best to go.
"Under his leadership the DA has shifted from a liberal ideology to an outright right-wing, conservative stance.
"He became a shop steward for big business and an enemy of the workers," Cosatu spokesperson Patrick Craven said.
Also pleased at his departure was the Independent Democrats' (ID) leader Patricia de Lille.
She expected it would intensify the ongoing battle between conservative and liberal elements within the DA.
"It certainly seems from the recent DA elections in the Western Cape that the conservatives are taking the DA over by force.
"If this is the case, the ID - which has always maintained that the liberals have been aggressively sidelined by a (former) National Party conservative majority under Theuns Botha - will have been vindicated."
Praise for Leon came from Inkatha Freedom Party (IFP) leader Mangosuthu Buthelezi, who described him as "a staunch patriot to the tips of his fingers".
His departure would "leave a considerable gap in our parliament and public life at a difficult time for our country".
"He has a supple ability to marshal words and ideas to sustain his case and then use them to devastating effect against his political opponents; and that, importantly, is how he saw those on the opposing benches: as opponents, not enemies."
Buthelezi had no doubt Leon's success would be measured "by the fact that his steadfast championing of our liberal democratic constitution and the ideals that are captured in our Bill of Rights will prevail long after his tenure of leader of the opposition expires". - Sapa