The proudest day of Neville Hymus' life turned to tragedy as the 67-year-old arrived at the palace in a taxi.
Pal Alexis Fairclough, who travelled to London with Neville from Liverpool, said: "He was so happy on the train.
"I know he was really looking forward to meeting the Queen. Neville became unwell when we arrived in a taxi at the palace gates.
"Despite the efforts of paramedics, he died at the scene. I would like to thank our taxi driver and the tourists who helped us."
Neville died at the gates of Buckingham Palace
Neville Hymus had been a lollipop man
The Queen and Prince Philip at the garden party
Alexis said that she was touched to hear that the Royal household contacted a London hospital to see how Neville was.
She said the palace contacted the hospital and news of Neville's death was passed to the Queen.
Alexis added: "Neville was an amazing, wonderful person. I am so proud of his charity work. But I am also so very saddened.
"He used to say that I was a 'sister' to him. I know Cancer Research UK was a cause close to his heart and I am sure he would want people to donate to the charity in his memory."
Neville working as a lollipop man
Neville was being rewarded for his charity work
Neville only gave up his job as a lollipop man when he was diagnosed with cancer
Neville's partner Eric Billington, paid tribute to his 'best friend in the world', who was also his carer: "I will miss him greatly."
Neville, from Waterloo, near Liverpool, patrolled the crossing in front of Merchant Taylor's Junior School in Crosby for several years and stood down last year after being diagnosed with cancer.
He was also a volunteer at community gardens in Waterloo and Crosby and created a memorial garden dedicated to those injured in WWI.
One Facebook tribute read: "Nev was a true gentleman and will be sadly missed. He was a fine example to others. RIP. I and others promise you and your work will not be forgotten."
Another read: "Such sad news. He was well loved by my children and such a lovely man. Thoughts are with his family."