Cliff Richard's No.1 most lucrative song revealed - and it might surprise you

For more than half a century, Derry-born songwriter Phil Coulter has been behind some of the most recognisable tunes in popular music. His name is attached to Eurovision winners, stadium anthems and moving ballads - but the song that has proved the most financially rewarding wasn't one of his Irish classics or even a rugby chant.

It was a Cliff Richard hit from 1968 that still brings in royalties today.

Speaking about the realities of the music business to the Belfast Telegraph, Coulter admitted that the track Congratulations has been his biggest earner. Written with frequent collaborator Bill Martin, the upbeat number was entered into the Eurovision Song Contest in 1968 and performed by Cliff Richard, already one of Britain's biggest stars at the time.

Although it only finished second in the competition - narrowly losing out to Spain's entry La La La - Congratulations went on to become a huge international hit.

It topped the UK Singles Chart for two weeks, sold more than a million copies worldwide, and quickly became a go-to song for weddings, birthdays and sporting celebrations. In Germany alone, sales reached around 150,000 copies. Its enduring appeal is what Coulter calls the mark of a true "pension song" - a piece of work that continues to earn royalties decades after its release.

Reflecting on its success, the 83-year-old said: "I always joke that Congratulations has fed, clothed and educated several of my children."

The songwriter, who raised six children with his wife Geraldine in Co Wicklow, has made clear that this single track has been the bedrock of his long career.

Coulter had arrived in London in the mid-1960s with little more than determination and musical talent, carving out a place for himself in the competitive world of Tin Pan Alley songwriting.

Before long, he was penning entries for the Eurovision Song Contest. Just a year before Congratulations, he and Martin had supplied Sandy Shaw with Puppet on a String, which went on to win Eurovision in 1967 and became the UK's first ever winner.

But while Puppet on a String gave him an early taste of international success, it was Cliff Richard's follow-up the next year that proved most valuable in the long term. As Coulter explained, every professional songwriter dreams of landing one of these rare works - a song that keeps working for you, long after its first rush of fame.

And Congratulations has certainly done that. The song was chosen as the centrepiece of Congratulations: 50 Years of the Eurovision Song Contest, a golden anniversary special that celebrated the competition's legacy. Richard himself also performed it during the commemorations for the 50th anniversary of VE Day in 1995, despite the track being written long after the end of the Second World War.

Its influence has even touched rock royalty. George Harrison's It's Johnny's Birthday, featured on his 1970 album All Things Must Pass, was directly based on the melody of Congratulations. Coulter and Martin later filed a claim for royalties, and subsequent pressings of the record gave them credit for their contribution.

More recently, the hit has continued to reach new audiences. It appears twice in the second series of Squid Game, proving the tune still resonates more than half a century after its Eurovision debut.