Labour has promised to raise the minimum wage to £8 per hour, if it wins the next election.
Leader Ed Miliband said it was "not good enough" that one in five people in the UK were on such a low wage.
The pay rise would be introduced by the end of its first parliamentary term in government, said Labour.
But the government said it had already delivered an above-inflation increase as, from October, the minimum wage will rise from £6.31 to £6.50.
The minimum wage for adult workers was £5.80 at the time of the last election.
Paul Kenny, GMB general secretary, said: "This is a welcome and necessary first step for workers to recover the nearly 15% drop in the value of earnings they suffered over the last six years.
“It is important to shift the burden of dealing with low pay from taxpayers to the employers many of whom are sitting on record levels of cash and profits. The transition must be real so that the increase is not paid with one hand and taken away with the other."