The M4 was closed today while traffic cops rounded up dozens of escaped pigs which spilled from an overturned lorry.

The animal transporter was travelling westbound on the M4 when it flipped on to its side, shedding 110 animals across the carriageway.

At least 13 pigs were killed in the initial accident before police, vets and rescue teams spent several hours treating and recapturing other animals.

Motorists faced 20-mile tailbacks and delays of up to three hours after the westbound carriageway was closed between junctions 17 and 18 at 10.30am.

A Highways Agency spokeman said: "With incidents like this it can be difficult to predict how long it will take to clear up.

"Obviously we've got to get all the pigs off the carriageway.

"The emergency services are the scene so they're leading that part of the operation and doing their best to round them up. We do advise people to avoid the area if possible."

The accident happened between the Leigh Delamere services in Wiltshire and junction 18 of the M4 near Bath.

A second animal transporter was taken to the scene to remove any surviving animals.

The police spokesman said: "At 10.22am a passing motorist reported that a lorry had overturned on the M4 and that there were animals loose on the carriageway.

"All three lanes were blocked by the accident and we are currently in attendance at the scene. We do not believe any other vehicles were involved in the accident.

"We have now managed to off-load most of the pigs into a new lorry but some of them died in the initial crash.

"One lane was reopened at midday but that will have to be closed to enable us to remove the lorry."