Mystery surrounds the runaway lorry that careered down the M1 through Hertfordshire near Borehamwood and Elstree at 80 mph.

Potters Bar lorry driver Michael Rayner was hailed a hero after he brought his truck to a standstill a mile from the end of motorway after the accelerator had jammed.

But Scania, the manufacturer of the lorry, said yesterday that an investigation into the incident revealed the speed limiter, which should have kept the speed of the lorry down, had been tampered with. The company also says that a non-Scania engine-speed limiting device damaged the truck's throttle linkage, forcing the accelerator to jam.

But both the speed limiter and the braking system were found to be working properly. An investigation has been launched by Metropolitan Police into the incident.

Mr Rayner, 26, from Little Heath, near Potters Bar, was driving the 38 tonne articulated truck southbound on the M1 on Sunday when his accelerator jammed just after reaching junction 10 at Luton airport.

He frantically pressed the brakes which fused the brake pads, putting them out of action, and dialled 999 on his mobile phone. Hertfordshire police sent a car and a helicopter to join him at Hemel Hempstead.

PC Patrick Jones of Hertfordshire traffic police, who was in a car which joined Mr Rayner at junction 8, said: "We tried to get the rest of the traffic out of the way so that he could use the whole motorway."

The police control room gave advice to Mr Rayner as he was drove, but PC Jones said the driver's mobile phone would occasionally cut out. "When that happened, we would pull up beside him and I would shout advice to him out of my window. I was trying to reassure him all the time as he was panicking a bit," he said.

The police told Mr Rayner to switch off the engine or put the gears into neutral so that the lorry could slow down.

But PC Jones said: "As he had not been driving HGVs for very long, he did not know what would happen if he did either of those things. He knew that if he switched off the engine there was a chance that the power steering would fail and that was something that he did not want."

As the truck passed Scratchwood Services PC Jones told Mr Rayner he had no choice but to turn the engine off. "We were only one or two miles away from the Staples Corner roundabout and the consequences of a 38 tonne lorry crashing into that does not bear thinking about," he added.

Mr Rayner pulled over on to the hard shoulder, switched the engine off as he drove into the crash barrier, and eventually came to a halt. He had travelled 20 miles and escaped unhurt, although he was badly shaken.

National newspapers revealed this week that Mr Rayner served five years in jail for physical assault on a toddler. The reason given at the trial was that he was suffering from the attention seeking disorder Munchausen's syndrome by proxy.

Mr Rayner denies he suffered from the disorder and that he tampered with the controls of the lorry, causing the scare.

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