At the time of the 2010 general election, the Conservatives promised a transferable tax allowance worth around £150 to married couples.
The Liberal Democrat leader Nick Clegg ridiculed the proposals as "patronising drivel that belongs in the Edwardian age".
His party later secured a provision in the coalition agreement allowing them to abstain from voting on the issue.
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Charities warn that sitting down too long can lead to health risks such as type 2 diabetes and cardiovascular disease.
The Labour leader will use a speech to set out the differences between his party and the Conservatives with particular emphasis on the NHS.
Kate will join Prince William on a tour of the Stephen Lawrence Centre before visiting a charity working with disadvantaged young people.