In the R&D programme for the ICARUS experiment, we have developed a practical procedure to purify liquid argon in the liquid phase. Extreme purity is obtained, corresponding to an electronegative impurity concentration below 0.1 ppb of 02 equivalent. This corresponds to an electron lifetime in the range of several milliseconds equivalent to attenuation length of a few metres. The procedure has been tested up to flows of the order of 1000 litres of liquid per hour. The new technique makes it possible to operate very large amounts of ultrapure liquid argon as foreseen for the multikiloton ICARUS experiment at Gran Sasso.