Sophie Stock, 20, had no idea her partner of five years Lawrence Key, 19, had popped the question until they climbed off the ride and collected their souvenir photo.
Romantic Lawrence had persuaded his scheming pals to hold up pieces of paper spelling out 'Will you marry me?' with an arrow pointing to Sophie sitting in the front of the flume.
As soon as the stunned garden centre worker clapped eyes on the picture - smitten Lawrence dropped on to one knee and pulled out his grandmother's ring in the middle of Thorpe Park theme park, Surrey, on Saturday.
Overcome with emotion, Sophie said yes before Lawrence swept her off her feet and whisked her away to Rome for a romantic three-day city break.
Stunned Sophie had no idea Lawrence had been planning the imaginative proposal
- and the couple had never even discussed getting married, even though they live together.
The secondary school sweethearts today revealed Lawrence's incredible relief that Sophie said 'yes' - because he had been planning the proposal for two years.
Bride-to-be Sophie, who jetted back from Rome, said: "I was really surprised when I saw the picture.
"I am really happy and excited to be getting married."
Sophie, who works at a home and garden store, and Lawrence, a bakery worker, met at Ralph Sadleir Middle School, Herts., and have been together for five years.
The couple now plan to wed on August 21, 2012, emotionally on the seventh anniversary of the death of Sophie's father, Grant Stock, who died from cancer aged just 45.
[Related article: Bride-To-Be Serenaded By Train Passengers]
Sophie's proud mother Sally Stock, 44, who helped put the plan into action, said: "He asked my permission to marry her which I thought was very nice about a month before.
"I was quite excited about it knowing it was going to be such a surprise."
Sally, a civil servant, drove Lawrence up to Norfolk to Sophie's grandmother Shirley's house to collect her ring to use for the proposal.
She added: "He had it in his jacket and, even thought it was really hot, he kept it on all day because he was too scared of loosing it.
"They nearly had a row because she kept saying 'why don't you take it off'.
"I was waiting for the phone call all day but she didn't call untill about 3pm because I think she needed to calm down."