Du Plessis was elected to the board in December

Du Plessis, who managed the team that won the 1995 World Cup, resigned amid attempts to revamp South Africa's entire rugby structure.

He is the latest top official to quit in the wake of the Springboks' poor display at the Rugby World Cup.

Du Plessis cited his relationship with South African Rugby Union president Brian van Rooyen for his departure.

"My nature is not confrontational but I have spent the past two months in an extreme state of confrontation with the president," said Du Plessis.

"This is not good for any of us, or for the game. I have agonised over this decision for countless sleepless nights... In the end I have to follow my conscience."

Du Plessis was one of six new members of the 14-man board elected at the South African Rugby Football Union (SARFU) annual meeting in December.

Among the rumoured changes discussed at a think-tank at the weekend were the dilution of the newly created elite Currie Cup competition.

The switching back of control of the Springboks from SA Rugby - formerly the professional arm of the game in South Africa - to SARFU, the governing body and amateur arm, was reportedly another possibility.

Respected figure

"Mr du Plessis has indicated in a letter to the chairman of the board Dr Theunie Lategan that he could not guarantee what his level of contribution would be in future following the suggested changes," the a statement on the SARFU website said.

Lategan added: "Morne is a well respected figure in the South African game and made a valuable input during his short involvement on the board.

"He has provided his reasons for his decision and we respect his position."

Springboks captain Corne Krige, coach Rudolf Straeuli, SA Rugby managing director Rian Oberholzer and SARFU president Silas Nkanunu have all quit since the World Cup in November.

South Africa were beaten 29-9 by New Zealand in the quarter-finals.

SA Rugby was hit by another scandal in January when financial auditors found that one million rand (�78,000) had been stolen from its coffers.

Two staff members have subsequently been suspended.