UPDATE 18.20: Traffic remains stop-start for four miles eastbound between junction 29 (A48M) and junction 26 (Malpas). The A48M before junction 29 is also still affected.

UPDATE: 3.52pm

All lanes have now re-opened.

UPDATE: 3.52pm

The motorcyclist's injuries are described by Gwent Police as serious.

UPDATE: 3.30pm

The road is now clear, but traffic is very heavy, and the backlog is affecting link roads.

Stop-start traffic is reported fro 12 miles eastbound between junctions 32 (A470, Coryton Interchange, Cardiff) and 26 (Malpas, Newport).

Traffic is also heavier than usual and very slow on the A4232 link, southbound between Cardiff Gate and Pontprennau.

And the A48(M) eastbound has slow traffic from junction (M4) back to junction 29a (St Mellons).

UPDATE: 1.57pm

UPDATE: 1.20pm

Two lanes are still closed. Travel time between J29 and J32 is between 90 and 120 minutes. 

UPDATE 1.15pm

The Welsh Ambulance Service said they were called at around 11.35am to reports of a road traffic collision on the M4 eastbound near Castleton.

Two paramedics in rapid response vehicles and a crew in an emergency ambulance attended the scene and an elderly man was taken to the Royal Gwent Hospital with potentially serious injuries.

UPDATE: 12.48pm

Two lanes are closed and queuing traffic is for eight miles on M4 eastbound at J29 A48(M).

Congestion goes to J32, A470 (Coryton Interchange) and on A48(M) to J29a, A48 (St Mellons).

12.40pm

A CRASH involving a motorycle and a number of vehicles has taken place on the M4 near junction 28.

Gwent Police were called at 11.33am this morning with reports of a crash on the eastbound side of the M4 just after junction 28. 

A force spokeswoman said: "Please expect delays whilst officers are at the scene dealing with the incident."