The Blades confirmed the news in a statement on their official Twitter feed.
It read: "Our thoughts are with former player Peter Ndlovu, who has been critically injured in a car accident in his native Zimbabwe. £sufc."
A statement from Birmingham read: "The thoughts of everyone associated with Birmingham City Football Club are with Peter Ndlovu and his family following news of a serious car crash in Zimbabwe.
"Peter was a very popular player during his time at St Andrew's and we hope he pulls through this difficult time."
Ndlovu moved to England in the summer of 1991 to join Coventry. He made 196 appearances in all competitions for the Sky Blues, scoring 43 goals.
Ndlovu moved on to Birmingham in 1997, making a further 137 appearances and netting 28 times for the Blues.
He joined Huddersfield on loan over the winter of 2000-01, scoring four goals in six games for the Yorkshire club, and left Birmingham on a permanent basis in February 2001 when he joined Sheffield United on a free.
He played 154 times for the Bramall Lane club, scoring 29 goals, and left them in July 2004 when he returned to African club football.
He was capped 100 times by Zimbabwe.