Alexander McKellar, 31, pleaded guilty to causing the death of Tony Parsons by hitting the 63-year-old with his vehicle on the A82 near Bridge of Orchy, Argyll and Bute, on September 29, 2017.
He pled guilty to culpable homicide and attempting to defeat the ends of justice.
He and his twin brother Robert McKellar both appeared in the dock at the High Court in Glasgow, where both also admitted a charge of attempting to defeat the ends of justice by burying Mr Parsons’ body.
While originally charged with murder, advocate depute Alex Prentice KC announced the charges against the two brothers had been amended.
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Mr Parsons, a former naval petty officer, was reported missing in 2017 while on a charity cycle from his home in Tillicoultry, Clackmannanshire, to Fort William, Highlands.
His remains were discovered in January 2021 in a remote area of ground close to a farm near the A82.
The brothers pretended that damage caused to the vehicle that struck Mr Parsons had been caused by a collision with a deer.
Prosecutors said the twins had attempted to defeat the ends of justice between September 29 2017 and January 3 2021, by digging a grave on the estate, depositing his body there, covering it with animal remains and bleach and burying him.
His family were at the High Court in Glasgow to hear the guilty plea.
The pair will be sentenced on Friday.