The article provides a conceptualisation of the link between recent migration flows and labour market uncertainty through the analysis of a critical example, the construction sector (characterised by economic volatility, worker mobility, employment insecurity, safety risk) in the UK and Spain (countries with large immigration, flexible labour markets and volatile construction sectors). Transnational labour mobility can be seen as a structural response to recent European dilemmas on how to combine flexibility and security, through the creation of a hyper-flexible buffer of migrant workers who, being disposable in case of downturn, can carry most of the uncertainty burden without causing political problems. This raises two issues: the social sustainability of such segmentation, in particular with regard to occupational health and safety, and the role organised labour can have on it, in particular in organising such workers. The issues are analysed through labour market statistics and interviews with unionists, migrant organisation representatives, employers and employment policy officers in both countries.