The size of atmospheric Particulate Matter (PM) is important as a determining factor for how long the particle stays in the atmosphere, and where it deposits in the human respiratory tract. Therefore, it is important to analyse PM2.5/PM10 ratios as an indicator of the fine particles and determine how the ratios vary both in space and time. This study uses the most recent 5 years (2010–2014) PM2.5 and PM10 data (µg m–3) from 46 monitoring stations, which are part of the UK Automatic Urban and Rural Network (AURN). In this paper mostly robust statistics, which are not sensitive to non-normal distributions and to extreme values in both tails of the distributions are applied to assess temporal trends in PM2.5, PM10 and their ratios. PM2.5/PM10 ratios demonstrated considerable temporal and spatial variability in the UK and 5 years median ranged from 0.4 to 0.8, resulting in overall median of 0.65. Theil-Sen temporal trend analysis showed that PM2.5/PM10 ratios have increased at several monitoring sites in the UK despite the fact that both PM2.5 and PM10 levels have predominantly decreased. However, trend in PM2.5/PM10 ratios averaged over the 46 monitoring sites was insignificant. Trends in the ratios of PM2.5/PM10 varied during different seasons: spring showed positive significant trend and winter showed negative significant trend, whereas trends in autumn and summer were insignificant. For further investigations: (a) Trends are adjusted for meteorological effect; (b) The emissions of PM10 and PM2.5 (kilotonnes year–1) and their ratios from 1990 to 2013 are analysed; (c) Temporal trends of the secondary particles (nitrate and sulphate) are analysed from 2000 to 2014; and (d) The diurnal, weekly and annual cycles in the ratios of PM2.5 and PM10 are analysed.