Ed Miliband has promised his party will look into any new evidence regarding allegations of vote-rigging in Falkirk, but refused to comment on whether the inquiry would be reopened.
The Labour leader admitted there were "machine politics" at play in Falkirk, but insisted his party are "learning the lessons" from the scandal.
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Ms Leadsom has received the backing of Ian Duncan Smith, the former work and pensions secretary.
The former shadow work and pensions secretary is considered a more viable candidate than Angela Eagle, a Labour source has told ITV News.
As many as 10 people have been rescued following the raid, according to a Bangladeshi official.