Firstly, of course, Leave.EU don't provide a source for the claim. The source is the latest ONS population digest:
Looking at the estimates by country of birth, between October to December 2015 and October to December 2016:The inference that Leave.EU make is that the reasons for this decline in the number of UK born people working is migration - 'British workers paying the price for mass migration'. And this could, indeed, be the reason for the decline. The problem is that neither the ONS data not Leave.EU provide the information needed to establish whether or not migration does put UK born people out of work.
- UK born people working in the UK decreased by 120,000 to 26.37 million
- non-UK born people working in the UK increased by 431,000 to 5.54 million
And there are other equally plausible reasons for the decline such as 'baby boomers' retiring, the decline in public sector employment leading to early retirements, increased numbers of people staying in full-time education, more women opting to remain as housewives. Unless all these possible reasons - and maybe others I've not thought of - are shown not to be the reason, we cannot make the claim that Leave.EU make. It is, regardless of the source's quality, a falsehood.