The move would achieve better production utilization at VW's Brussels plant and avoid the high investment required to increase capacity at Audi's own factories.
Audi is introducing its new five-door A3 Sportback in September. The launch will push Audi's Ingolstadt plant beyond its capacity.
Audi chief Martin Winterkorn said the company is aiming to increase A3 production to 1,000 cars a day. Ingolstadt has capacity for 700 of these.
"We will not be able to build the additional vehicles within normal working hours", said Xaver Meier, head of Audi's works council. This is why additional A3s will be built in Brussels."
Winterkorn said there is further capacity at Brussels if demand for the A3 increases in 2005.
The five-door A3 Sportback will be built in Ingolstadt. From August around 300 three-door A3 models will be built a day in Brussels.
VW produces the new Golf in Brussels. The Golf shares many components with the A3.