Shoppers spent £4.98bn online during the period, compared with £3.3bn for the same time a year earlier, according to e-commerce trade body IMRG.
For 2005 as a whole, it calculated that spending over the internet in the UK totalled £19.2bn, 32% more than 2004.
Overall retail sales rose by 4% in December compared to the same month in 2004, official figures have shown.
The figure from the Office for National Statistics was in line with expectations.
IMRG managing director Jo Tucker said the 50% growth in online sales represented a "step change in retailing at Christmas".
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