Global PC shipments this year are expected to plummet by nearly 12%, marking the steepest drop in the industry's history, a market research firm said Monday.

Driving shipments down are users holding on to PCs longer, which has led to manufacturers cutting production. "The PC industry is facing extraordinary conditions as the global economy continues to weaken," George Shiffler, research director of Gartner, said in a statement.

Shipments this year will fall 11.9% from 2008 to 257 million units, the analyst firm said. The drop easily beats the previous worst decline, which was 3.2% in 2001 following the dot-com bust.

The unprecedented market slowdowns will occur across emerging and mature markets. In the former, shipments are expected to fall 10.4%, while mature markets are expected to see a 13% decline. In both markets, a slower gross domestic product, or GDP, will generally weaken sales. With people and businesses expected to postpone replacing old machines, PC makers will keep inventories at "historic lows until confidence in a recovery eventually firms," Shiffler said.

"The impact of reduced replacements will be especially acute in mature markets, where replacements are estimated to account for around 80% of shipments," the researcher said.

Shipments of desktops will drop 31.9% this year to 101.4 million units. Mobile PC shipments, on the other hand, will grow 9% from last year, substantially boosted by growth in the mini-laptop market. Excluding the latter systems, also called netbooks, other mobile PC shipments will grow just 2.7%.

Shipments of mini-laptops will rise to 21 million units from 11.7 million 2008, Gartner said. Netbooks, defined as sub-$500 systems with screen sizes 10 inches or less, are expected to represent 8% of PC shipments this year.