The UK Statistics Authority has written to Labour MPs pointing out that they haven't been presenting statistics on zero hour contracts entirely clearly.
Contrary to Labour's suggestions, 1.8 million people are not 'on zero hour contracts' but rather around 1.8 million zero hour contracts were in use in August 2014.
The number of people with a zero hours contract in their main job was estimated to be 697,000 in October-December 2014. The discrepancy is explained by the facts that people can have more than one zero hours contract or have a zero hours contract as well as a different main job.
The ONS have pointed out that although the number of zero hour contracts estimated to exist rose from 1.4 million in January 2014 to 1.8 million in August, this may be due to seasonal variations. It certainly doesn't imply that the number of people on zero hours contracts increased by 400,000.
For a fuller explanation of the facts about zero hour contracts, see our spotlight on the topic (here).