Jack Hurst, who is aged just 18, from Desford, Leicester, has been appearing on the show throughout the past week.
On Friday's episode, his eighth and final appearance on the show, he scored a massive 946 points – the highest ever achieved in the show's 28-year history.
During his performance, Jack found six nine-letter words and won each game with a clear majority.Speaking to the Daily Mail, he said: 'It was a really good feeling.
'After Wednesday's show a producer told me I only needed 114 points in the last two shows to get the record.
'Then on Thursday's show I got 133, which was the highest score in the series.'
Jack beat research scientist Jaonathan Welti on Friday's show with words such as 'potatoes', 'petunias' and 'idolise'.
The Cambridge maths student, who filmed the episodes earlier this month, finished top of his class at Bosworth Community College in Leicestershire after achieving four A* grades in his A-Levels.
Next month he will return to Manchester to film the grand final, in which the top eight players of the year will compete for a 20-volume gold-leafed dictionary set.
Jack said: 'I'm actually going for second place because the runner-up gets £1,000 and I'd rather have that.'
The entire Hurst family has been excited by Jack's amazing performance. His brother, Josh, 22, a geography student at Manchester, said:
'He came across as quite funny on TV and as having a really good personality.
'He's been watching the programme for about seven years and he's always practicing on the Countdown website, so we knew he would do well.'
Grandad John, 79, said: 'It was fabulous. We're very proud of him. He's always been very smart.' The previous record score was 930, amassed by University of Warwick economics student Andrew Hulme in July, 2009.
In December 2002, teenager Julian Fell hit the headlines when he racked up a single-programme record score of 146.
A spokeswoman for Granada, which produces the show, said: 'Jack was a terrific contestant with a great personality as well as a great brain, and he has every chance of winning the final on December 17.'
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