Separately, the not-for-profit Resolution Foundation think tank warned the UK's decision to leave the EU could affect the government's new wage policy.
It said the so-called Brexit would likely reshape the landscape in which many low pay sectors operate, creating huge uncertainty about the outlook for earnings over the coming years.
Weaker wage growth, it said, could reduce the current projected real terms value of the NLW by up to 40 pence an hour by 2020.
The policy was announced in last summer's Budget by Chancellor George Osborne, in what he said was a move to create a higher-wage, lower-welfare economy.