Any diamond manufacturer found to have purchased stones that fuel wars in Africa will no longer be able to buy from De Beers, which controls 60 per cent of the world's uncut diamonds.

The South African company's move is a response to mounting international criticism and to fears inside the industry that diamonds could face a consumer boycott similar to the one that hit the fur industry.

De Beers, chaired by Nicky Oppenheimer, argues the policy will drive a wedge between the legitimate and illegitimate sides of the industry, forcing manufacturers, mining groups and jewellers to follow. "We want to get to the point where consumers have the choice of going to guaranteed jewellers, and pay a full price, or go to discounted jewellers that cannot offer a guarantee that their diamonds do not come from conflict areas," a person close to De Beers said.

De Beers' stance comes as the company gives up its long-standing monopoly on uncut diamonds and demands that the industry double its advertising budget.

It bowed to shareholder pressure this year and stopped buying surplus diamonds to control prices and started selling part of its $4bn-diamond stockpile.

De Beers wants an industry clampdown in order to avoid a public backlash that would reduce demand and make it more difficult to sell the company's stocks. The diamond industry has been under intense pressure from lobby groups and a UK government-led campaign to prevent rebel movements in Angola, Congo, and Sierra Leone using the proceeds of diamond sales to buy weapons.

The United Nations security council last week agreed an embargo on diamonds mined by the Revolutionary United Front, the Sierra Leone rebel group. The ban mirrors a measure imposed against Unita, the Angolan rebel movement last year.

De Beers issued a guarantee last March that its diamonds are not from a conflict area.

But at a meeting in London on Wednesday, De Beers will tell its 125 "sightholders" - the select group of clients that buys its rough stones at 10 annual "sights" - to demand similar guarantees when buying in major trading centres such as Antwerp, Bombay and Tel Aviv.

Sightholders will be told that if they buy diamonds without a written guarantee that they are "conflict-free", they could lose the coveted right to buy De Beers' uncut stones.

The pledge is part of an overhaul of the relationship between De Beers and its clients. Commercial transactions between the company and the sightholders have been based on gentleman's agreements for 70 years. But the South African group will formalise the relationship by asking clients to agree to a set of rules.

De Beers currently accounts for the bulk of advertising with its "A diamond is forever" campaign. The company spent $170m on marketing last year.

It wants to double the industry advertising budget by asking manufacturers and jewellers to pay for marketing specific products.