Composer Paul Harvey, 80, from Sussex, improvised tune using just four notes.
Went viral and turned into full orchestral piece by BBC Philharmonic Orchestra.
Paul heard it played for the first time yesterday and aired on BBC Breakfast today.
Broke down after receiving video message from his hero Stephen Sondheim.
BBC Breakfast viewers were left 'in bits' this morning after a pianist with dementia who improvised a tune from just four notes which went viral heard his arrangement played the BBC Philharmonic Orchestra.
Paul Harvey, 80, from Sussex, was visibly overwhelmed after hearing the moving piece - and broke down in tears after he was surprised with a video message from his hero, fellow composer Stephen Sondheim.
The emotional segment moved hosts Louise Minchin and Dan Walker to tears, while dozens of viewers took to Twitter admitting they were in floods at 6:25am.
'Paul's Tune' will be released as a charity single next week with the proceeds split between two charities - the Alzhiemer's Society and Music for Dementia.
Former concert pianist and inspirational music teacher Paul, who counts Cutting Crew's Nick Van Eede among his pupils, was diagnosed with dementia late last year, but still manages to play music from memory - and compose new pieces.
Last month he was challenged by his son Nick to come up with a melody using just four notes - an old party tricky of his.
Nick picked F natural, A, D and B natural for his father to play and, dressed in red and white striped pyjamas, Paul improvised a beautiful piece.
A clip of his performance - intended to show how musical ability can survive memory loss - went viral and was played on BBC Radio 4's Broadcasting House to mark World Alzheimer's Day on September 21.
Listeners were so moved by the piece that they asked the BBC to have it fully orchestrated - which is when David Whilbey, of the BBC Philharmonic Orchestra, stepped in.
'It's a wonderfully nostalgic, romantic piece of music and the fact that it was improvised as well makes it more special really,' he said.
'It really just highlights the power of music, and the importance of music to mental health.'
Dan adapted Paul's tune for full orchestra, with individual parts recorded by musicians while they were locked down at home. It was played on Paddy O'Connell's show yesterday.
Speaking on BBC Breakfast this morning, ahead of the duo hearing Paul's Tune for the first time, Nick said: 'Dad is at heart a performer and an entertainer and a pianist, and when he sits at the piano, and either performs a classical piece or improvises, it brings him back. It's vitally important in his life.'
After hearing his piece, Paul said: 'Wow, wow. Fantastic. I was just listening to a wonderful piece of music and then all of a sudden I said to myself, "I wrote that! I wrote that tune!"
'Wow. I won't forget that, that was very wonderfully special.'
Speaking about his music heroes, Paul said his all-time idol is American composer and lyricist Stephen Sondheim.
'He writes the words and music, which is ridiculous, people shouldn't have that sort of talent, but he has,' Paul said.
Having contacted Mr Sondheim, BBC Breakfast treated Paul to a very special video message, which was broadcast today.
Flabbergasted, Paul watched as his hero appeared on the screen and paid tribute to his work.
'Hello Mr Harvey, this is Stephen Sondheim, and I just wanted to thank you so much for the compliments you gave me, and also thank you for that lovely tune that you invented, which I can't wait to steal,' he joked.
Almost sobbing, Paul gasped: 'Wow, thank you so much, wow, dear oh dear oh dear oh dear, I didn't think there was any more surprises, the great man himself, the great man himself, wow!
'Tell him, for him, he can have that tune with pleasure, any body else I'd charge for it. Wow, oh my goodness. Oh dear, that is deeply wonderful. Thank you so much for doing that.'
Following the segment, Dan Walker tweeted: 'All sorts of sobbing going on this morning after watching Paul and his son @mrnickharvey on #BBCBreakfast this morning. BEAUTIFUL.'
Dozens replied, including actress Kate Robbins who admitted: 'In absolute bits. What a wonderful arrangement by @daniel_whibley and executed so perfectly by @BBCPhilharmonic. Seeing Stephen Sondheim's tribute to Paul got me going again - just after I'd stopped bawling.'
James Ransom tweeted: '4notes from Paul, just beautiful, great way to start the day, music to move the soul.'
Now Paul and Nick hope his single will make it to Christmas number one.