It's on the loose...$12k engagement ring inside a helium balloon.

The luckless 28 year-old's dreams of giving his sweetheart, Leanne, 26, the ultimate proposal have literally vanished into thin air.

Hajji, of Hackney, east London, had concealed a $12,000 engagement ring inside a helium balloon. The idea was that she would pop the balloon as he popped the question.

But as he left the shop, a gust of wind pulled the balloon from his hand and he watched the ring -- and quite possibly the affections of his girlfriend -- sailing away over the rooftops.

"I couldn't believe it," he told The Sun newspaper.

"I just watched as it went further and further into the air.

"I felt like such a plonker. It cost a fortune and I knew my girlfriend would kill me."

Hajji spent two hours in his car trying to chase and find the balloon, without success.

"I thought I would give Leanne a pin so I could literally pop the question," he said.

"But I had to tell her the story -- she went absolutely mad. Now she is refusing to speak to me until I get her a new ring."

He is hoping the ring will still turn up.

"It would be amazing if someone found it," he added.