Millennial-scale variability in the flux of ice-rafted detritus to North Atlantic sediments during the last glacial period has been interpreted to reflect a climate-forced increase in the discharge of icebergs from ice-sheet margins surrounding the northern North Atlantic Ocean(1). But the relationship between ice-sheet variability and climate change is not clear, as both the sources of ice-rafted detritus and the ice-marginal processes are varied and complex(2-4) Terrestrial records are helpful in unravelling this complexity because they can demonstrate the scale of ice-sheet oscillations, and whether the ice sheet (or sector) was advancing or retreating with respect to climate change. Here we constrain the age and anatomy of a prominent readvance of the British Ice Sheet in the northern Irish Sea region at similar to 14(14) C kyr BP (similar to 16.4 calendar kyr BP). The analysis indicates that the British Ice Sheet participated in an iceberg discharge episode known as Heinrich event 1. Comparison with other terrestrial and marine ice-sheet records suggests that the dynamic collapse of the Laurentide Ice Sheet beginning at 14.6-15.0 C-14 kyr BP1,4 (similar to 17.2-17.6 calendar kyr BP)(5) initiated varied responses from other ice-sheet margins around the northern North Atlantic region. These observations support the argument that the release of icebergs and meltwater during Heinrich event 1 disrupted the North Atlantic thermohaline circulation(6-8), leading to a delay or reversal of deglaciation of the Northern Hemisphere and at least as far south as 40 degrees S for two to three thousand years(5,9,10), suggesting a climate forcing and response similar to that of the ensuing Younger Dryas `cold snap'(11,12).