Thousands of drivers have been caught in M40 tailbacks up to 15 miles long for three mornings running - but highways officials cannot agree on the cause.

The queues on the southbound carriageway have stretched from junction 9, where the A34 meets the motorway at Wendlebury, near Bicester, as far north as Banbury.

Highways Agency officials insist the traffic lights at junction 9, where the M40 meets the A34, are not responsible, and claim roadworks on the A41 between Bicester and the M40 are causing the delays.

But Oxfordshire County Council highways officers say the lights on the roundabout at the junction have been malfunctioning and are responsible for the jams.

Martin Dellar, a spokesman for the Highways Agency, said: "A team of engineers have checked the phasing of the lights and have confirmed that they're working properly.

"Traffic was heavy on the A41 between Bicester and the M40, where it's down to one lane at the moment because of county council roadworks.

"There were no accidents in the area which would have contributed to the hold-ups.

"Our contractor, UK Highways, which looks after the M40 on our behalf, has reported tailbacks of up to 15 miles every day this week, and it must be hugely frustrating for drivers. The roadworks on the A41 are a major contributing factor."

David Robertson, the county council's cabinet member for highways, said: "Our own engineers have reported back to us that the lights on the roundabout at junction 9 were out on Tuesday, and that was causing a major problem for drivers. They say the traffic on the A41 is not contributing to any tailbacks.

"Traffic is down to one lane on the A41 between junction 9 and the Tesco roundabout in Bicester for resurfacing and we're introducing a convoy system to slow traffic, which protects staff working on the road."

The resurfacing work is due to be completed by Sunday.

Mr Robertson added: "I will make sure that our engineers have detailed talks with the Highways Agency so that between us, we can solve the problem and ensure that drivers don't suffer long delays on their way to work."