Sir Paul McCartney has paid tribute to Sir George Martin, the record producer known as the Fifth Beatle, who has died aged 90.
Sir George helped The Beatles achieve global success while head of the Parlophone record label after hearing their demo tape in 1962.
His family confirmed he died peacefully at his home as tributes poured in from the world of music and beyond.
In a statement, Sir Paul said: "I have so many wonderful memories of this great man that will be with me forever. He was a true gentleman and like a second father to me.
"He guided the career of The Beatles with such skill and good humour that he became a true friend to me and my family. If anyone earned the title of the Fifth Beatle it was George."
Sir Paul described Sir George as "the most generous, intelligent and musical person I've ever had the pleasure to know" and recalled his favourite memory of the producer, when he persuaded the singer to include strings on The Beatles' classic hit Yesterday.
"When we recorded the string quartet at Abbey Road, it was so thrilling to know his idea was so correct that I went round telling people about it for weeks," Sir Paul said.
"His idea obviously worked because the song subsequently became one of the most recorded songs ever with versions by Frank Sinatra, Elvis Presley, Ray Charles, Marvin Gaye and thousands more.
"The world has lost a truly great man who left an indelible mark on my soul and the history of British music. God bless you George and all who sail in you!"
Ringo Starr paid tribute to Sir George for his "love and kindness" to the Fab Four through the years.
The former Beatle tweeted: "God bless George Martin peace and love to Judy and his family love Ringo and Barbara. George will be missed."
He also posted a picture of the Fab Four and Sir George with the caption: "Thank you for all your love and kindness George peace and love."
Sir George's son Giles, who is also a producer and has worked at Abbey Road studios, wrote: "RIP dad. I love you. I'm so proud to have been your son. I'll miss you more than words can say. Thank you for the all times we had together."
Sir George's manager Adam Sharp described him as "a true gentleman to the end" .
In a statement, he said: "The family would like to thank everyone for their thoughts, prayers and messages of support.
"In a career that spanned seven decades, he was an inspiration to many and is recognised globally as one of music's most creative talents. He was a true gentleman to the end."
Sir George took The Beatles from their early 1960s pop recordings to their classic albums Revolver and Sgt Pepper's Lonely Hearts Club Band.
A carpenter's son from Holloway in north London, he studied at Guildhall School of Music and played the oboe professionally before joining the recording industry.
During his career, which spanned seven decades, he won two Ivor Novello awards, six Grammys and in 2008 was the recipient of the Grammy Foundation's Leadership Award which saluted his humanitarian work.
In 1965 he was nominated for an Academy Award for his work on the music in A Hard Day's Night and in 1984 he received the Brit award for outstanding contribution to music, having been named best British producer at the first Brit awards in 1977.
Following the 1970 break-up of The Beatles, he worked with a host of artists including Sting, Celine Dion and Jeff Beck. He also produced two James Bond themes - Shirley Bassey's Goldfinger and Live And Let Die by Paul McCartney and Wings.
He received a knighthood in 1996 and a year later produced Elton John's re-write of Candle In The Wind for the funeral of Diana, Princess of Wales, which went on to become the biggest-selling single of all time.
In 2002 he was part of the team which organised the Queen's Golden Jubilee concert at Buckingham Palace, an event headlined by Sir Paul.
Paying tribute to Sir George, Sean Ono Lennon, son of John and Yoko, posted a picture of him on Instagram with the caption: "R.I.P. George Martin. I'm so gutted I don't have many words."
Prime Minister David Cameron tweeted: "Sir George Martin was a giant of music - working with the Fab Four to create the world's most enduring pop music."