ADM Milling said soaring energy prices combined with a disappointing wheat harvest meant it would be forced to add £28.75 to the price of a tonne of flour from 11 September. Experts said that this would equate to a 15-20% increase for a major user.
Ian Pinner, ADM Milling managing director, said: "This is the third year that we have had to accept increased energy costs.
"In addition, global wheat markets have moved up sharply in reaction to supply and demand figures showing world stocks to be the lowest for 25 years. Weather conditions have had an adverse effect on crops.
"These global factors have pushed up UK wheat prices to a level 15% higher than in November 2005."
The size of the flour price hike was far larger than would normally be expected, said Alexander Waugh, director general of Nabim, the trade association for UK flour millers. He predicted other leading millers would follow suit.
"It's unusual to see such a large rise, but there have been big hikes in the costs of energy and wheat.
"Year-on-year energy prices have risen by 60%, and wheat yields have been affected by the hot weather, not only in Britain but also in France and Germany."
Price rises would feed through to consumers, he warned. "Ultimately I guess they will, otherwise someone in the supply chain would have to take a hit."
Gordon Polson, director of the Federation of Bakers, said bakers were also being affected by rises in fuel prices. He added: "If there are increases in flour costs it is inevitable that these costs will have to be recovered."
The flour milling sector is currently struggling to cope as competition intensifies in the bakery market. Of 60 mills in the UK, six have closed in the past two years, three have changed hands, and a further three are up for sale.