Boris Johnson told a parenting class on Friday that he had his Smarties taken away by his grandma when he left the door open.

He was visiting Parent Gym, based at Shaftesbury Park Primary School, Battersea, to encourage more parents to volunteer to be coaches for the scheme.

It is the only parent gym in Wandsworth, and has been running for two years.

The organisation wants to open in 66 Wandsworth schools and Mayor of London wants to extend the scheme to 1,000 schools across the capital by 2013.

Mr Johnson said: "The Parent Gym is good because it is run by parents helping other parents, and through Team London people can volunteer to do what the parenting coach here is doing.

"It is a good system and the parents seem to enjoy it. It is a great way for people who have free time during the week to build their skills."

Mr Johnson sat in on a session and helped to develop a reward chart with 12 parents as well as sharing some experiences from his childhood.

Parent coach Caroline Boyd, 38, said: "I have had a really good time. It is great experience and great for the CV. Before I was just at home and it has got me doing so much more."

Kelly Wholey, 30, whose six-year-old son Jamie Wholey goes to the school has been attending the gym since March.

She said: "It is not a parenting group that tells you that you are wrong, you can go to it and talk there with other people that have problems with their kids.

"My child listens to me more and now, and I speak to him more calmly."

The class is free for parents who volunteer to take part and runs one morning a week.

Company Mind Gym funds the project and volunteers can sign up at Team London's website www.london.gov/teamlondon