The last thing Jessica Poutney expected to take home from a shopping trip with her boyfriend was a diamond engagement ring.

The teenager thought she was meeting her boyfriend Damian Warren, of Courtenay Gardens, Harrow, to celebrate their one-year anniversary in the intu Watford centre on Friday.

Unbeknown to her, a crowd that was assembling on the ground floor was waiting for her partner to pop the question.

At about 4.15pm, Mr Warren, 26, walked out of Pandora with his 19-year-old girlfriend to watch youngsters from the Breakout Dance Company perform a special routine.

The 28 children danced to a dubstep track played over a public address system at the bottom of the steps.

The performers unzipped their hoodies at the end of the routine, and revealed the words "Jessica will you marry", with Mr Warren’s top reading "me?"

Jubilant crowds cheered for the happy couple as Ms Poutney accepted the proposal.

Speaking through tears and big smile, she said it was "completely unexpected".

Ms Poutney said she had known Mr Warren for years and had got back in touch after he appeared on the Channel 4 television programme The Undateables, a documentary about the exploits of the unlucky-in-love.

She added: "I started talking to him again after that and we went on a date and he then asked me out.

"We’ve been going out for a year and today we were meant to be celebrating our anniversary. I just can’t believe that my whole family knew about it and managed to keep it a secret.

"It’s absolutely amazing. I still can’t believe it."

Mr Warren’s mother, Julia, helped him organise the surprise.

She said: "It’s brilliant and I’m so proud of him. All these kids that performed the dance have been absolutely great and Damian’s been working so hard planning it all. I’m still shaking with excitement."

Vicki Shepherd, who choreographed the routine, said Mr Warren approached her about one month ago to arrange the flashmob.

The dance tutor said it was the first time she had arranged something like this but that she was really happy the routine was so well received by the crowds on this special and unique occasion.

Howard Oldstein, general manager of the centre, said: "I was amazed with the amount of people that turned up. A lot of work went into it and it’s great to be involved in it.

"As a nation I think we like to see people being happy and when something like this happens it’s something to be proud of."