Boris Johnson's godfather and the former Vice-Chancellor of Oxford University hit a motorcyclist whose leg later had to be amputated, a court heard.
Sir Colin Lucas seriously injured the biker who said his life had been ruined by the loss of a limb caused by the crash.
The professor, 81, who ran the university from 1997 to 2004, was on his way to a hair cut in Witney when he carefully overtook a cyclist on the A4095 in Bampton, Oxfordshire.
However, the court heard he failed to see former police officer Tyler Parsons' silver Honda motorcycle.
The nearside of Lucas' 69-plate Mazda CX5 hit Mr Parsons, and sent him off his motorcycle.
Mr Parsons, a former British Army soldier and Thames Valley and Dorset police officer, spent two weeks in intensive care after his right leg was so badly damaged surgeons needed to amputate it below the knee.
In a victim personal statement read to Oxford Magistrates' Court by prosecutor Liz Tweed, grandfather Mr Parsons said: "Being involved in this collision has been life-changing on many levels for both my wife and me."
Mr Parsons, who was working as a sales assistant for Go Outdoors in Swindon before the crash, said he previously had an active lifestyle, running, scuba diving every other weekend and regularly going on caravan holidays with his wife.
Ms Tweed told the court that on July 1 last year, a Mr Price was cycling along the A4095 from Bampton towards Witney when he was overtaken first by a white Mini then by the defendant's Mazda.
"[The defendant] overtook Mr Price on his bicycle but was in collision with an oncoming motorcyclist, Tyler Parsons, who was on the correct side of the road," she said.
Lucas of, Clanfield, Oxfordshire, pleaded guilty to careless driving.
He was fined £324, ordered to pay £139 in costs and surcharge and given six penalty points.
The court heard the defendant refused to answer questions when he was interviewed by the police on July 30.
Instead, he provided a prepared statement in which he said he had been driving at around 35mph when he overtook Mr Price,
Lucas thought he had returned to his side of the road when he heard a bang and lost his wing mirror, he said.
He looked in his rear view mirror and saw the stricken motorcyclist, stopped his car and went back to the scene.
Mitigating, Simon Macfarlane said his client was extremely remorseful and had written a letter of apology to his victim.
The former academic, who before leading Oxford University was Master of Balliol College, had no previous convictions and had held a driving licence since 1959.
The solicitor said Lucas had given Mr Price a wide berth: "The only possible explanation for how this situation arose is that in focusing his attention on the cyclist, ensuring he was overtaking him safely, he has simply not seen the oncoming motorcycle.
"This was an unintentional lapse of attention of the oncoming carriageway as he focused his sight on Mr Price, the cyclist, in his carriageway."
Historian Lucas is the godfather of Boris Johnson.
The Prime Minister was even sacked from The Times newspaper after making up a false quote and trying to pass it off as his godfather's.
Lucas made a complaint to the newspaper after seeing the comment.
In a later interview in 2002, Johnson described the episode as his 'biggest cock-up'.
