The consumer group claims that the company has sold an estimated 55,000 vehicles since it was made aware of the fault six months ago - with VW Polos, Seat Ibizas and Seat Aronas affected.
When all three rear seat belts are in use in affected cars driven at high speed, the rear left passenger side can become unbuckled.
The issue was discovered by a Finnish motoring magazine back in May, but Which? says many motorists were not told about the issue when they purchased their cars - and some of them have been using all of their rear seats.
Which? also believes that an interim fix, which involves the faulty seat belt block being temporarily secured with a plastic cable tie, does not fully stop the chance of the seat belt coming undone.
A permanent fix is being introduced on Monday, and Volkswagen Group says drivers will need to take their cars to a garage for the free-of-charge repair.
But Alex Neill, the managing director of home products and services at Which?, said: "VW's handling of this potentially lethal safety issue has been completely unsatisfactory.
"It's shocking that they proposed a permanent fix that doesn't even properly solve the problem, and we're concerned that customers might not always be getting the right information at the point of sale.
"The decision not to suspend sales when the problem was discovered has now put substantially more drivers, as well as their passengers, at risk. The DVSA (Driver and Vehicle Standards Agency) must investigate VW's handling of the whole situation."
A Volkswagen Group spokesperson said customers had been advised not to use the middle seat belt until the problem was solved.
"It is important to note that there have been no known cases of any seat belts being released in this manner, other than in highly specific and exceptional circumstances demonstrated in the test by a Finnish car magazine," they said.
"Given the limited circumstances in which the seat belt can (in the test conditions) come unbuckled, and the employment of the interim fix and further still the specific warnings provided to users, there is no materially increased risk.
"It is on that basis that sales continued."