One of the two deputy presidents of the Independent Democrats, Simon Grindrod, has defected to the newly formed Congress of the People.

Grindrod, who was also Cape Town's mayoral committee member for economic development, made the announcement at a media briefing flanked by Cope leaders in Cape Town on Thursday afternoon.

"I am here because every South African must grasp this opportunity which may only come around once in a lifetime to provide an alternative to the ANC," he said.

"I want to be part of making history: that is what is happening in our country at the moment."

He said the move was a "personal decision", and he did not want it to be portrayed as a criticism of ID president Patricia de Lille, for whom he had the greatest respect.

ID principles and policies were "appropriate", but the ID as a party was not yet at a stage where it could replace the ANC.

Asked about Cope's policies, Grindrod said he had discussed them with the party's interim leadership, and they would be clarified in time.

"I can assure you they're not offensive to any moderate-thinking South African who believes in the future of... their country," he said.

Grindrod said if South Africans wanted change they had to break the mould of voting according to colour.

Cope executive member Phillip Dexter, who along with Cope heavyweights Mluleki George and Leonard Ramatlakane flanked Grindrod at  the media conference, said Cope was working "flat out" to finalise draft policy documents.

George said draft agriculture and education policies had already been formulated.

Grindrod said he had resigned his mayoral committee post.

He had earlier on Thursday sent a letter to De Lille informing her of his decision, and though he had tried to meet her in person, it had proved impossible.

De Lille said in a statement that while the party was surprised and disappointed by Grindrod's resignation, the ID had many capable candidates to take over his duties.

She said Grindrod was the "second" of the ID's two deputy presidents, and the first deputy, Agnes Tsamai, would continue serving in that position.

"We wish Mr Grindrod well with his new party," she said.

De Lille formed the ID in 2003 when she walked out on the Pan Africanist Congress.