The collision, involving a Renault Clio and a Citroen Xsara, happened this morning at the Blaris on-slip on the M1 at Sprucefield around 1.20am.

One man was believed to have been very seriously injured in the incident, while three women sustained injuries which were not believed to have been life-threatening. The man and two of the women were taken to the Royal Victoria Hospital in Belfast for treatment, while one woman was taken to the Lagan Valley Hospital in Lisburn.

Police said that a 21-year-old man has been arrested for dangerous driving causing grievous bodily injury following the crash.

They have appealed for anyone who witnessed the incident or who has information which may be of assistance to contact police at Lisburn on 0845 600 8000 or Crimestoppers on 0800 555 111.

"The Blaris on-slip will remain closed until at least mid-morning and this will affect rush hour traffic on the M1, A1 and the Park and Ride near Sprucefield," a police spokeswoman said this morning.

According to Roads Service officials the M1 approaching Broadway was busy this morning. However, it is not thought that traffic was too badly disrupted on the surrounding roads or roundabouts.

Traffic on the M1 into Belfast is already subject to a number of restrictions due to ongoing roadworks on the Westlink. Traffic lanes are narrower than normal and the roads are subject to a reduced speed limit.

Meanwhile, the Laganbank Road in Belfast was closed this morning due to flooding in the area. This denied access between Oxford Street and East Bridge Street for traffic moving within the city centre.