But in a romantic twist, the tragedy led him to the love of his life.

Mr Costello, 31, is set to marry the nurse he met while being treated in hospital for his injuries.

He proposed in the French city of Lyon, while on a cruise organised for survivors, family members and first responders of the Boston bombing.

"I now realised why I was involved in the tragedy. It was to meet my best friend, and the love of my life,” Mr Costello is reported to have posted on Facebook.

“I’m happy to announce that we will spend the rest of our lives together," he said.

Mr Costello was standing near the finish line at the Boston Marathon on April 15, when a double bomb blast struck competitors, killing three people and injuring scores of others.

An image of Mr Costello, jeans in tatters and legs blackened by the explosion, was published around the world.

He was later hospitalised at Massachusetts General Hospital with burns so severe he required multiple surgeries and pig skin grafts on his right arm and leg.

He described the experience as “one of the worst days of my life” and, at the time, suffered not only physical injuries but emotional difficulties after learning about the conditions of his friends.

Two weeks later, after being transferred to Spaulding Rehabilitation Hospital, Mr Costello met nurse Krista D’Agostino.

"After sharing a handful of conversations I realised not only how beautiful she was but also what a kind heart she had," he said.

"I somehow convinced her to attend a benefit with me, which turned into a few dates, which turned into a few inseparable months."

Mr Costello and Ms D’Agostino, who discovered they shared mutual friends, have been dating for eight months.

Fairfax Media, with agencies