The Office of Fair Trading has launched an investigation into games on phones and tablets that are free to download, but could end up costing parents hundreds - even thousands - of pounds.
It follows concerns that children could be tricked into spending money on upgrades without realising the cost.
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The Labour leader did not say whether he would maintain Britain's nuclear deterrent programme.
Ukip leader Paul Nuttall also said he would want to get rid of Sharia courts.
Over 40,000 people picked up their packs to take part in the annual marathon in the capital.