The first electric "Desiro UK" units were ordered by South West Trains for both short- and long-distance service out of London Waterloo to Alton, Basingstoke and the Hampshire/Dorset coast, to replace its Mark 1 slam-door EMUs, which were due for replacement by the end of 2005, as most were more than 40 years old and did not meet modern health and safety requirements. They became two different classes, both of which use the 750 V DC third-rail power supply. One hundred and ten Class 450 four-car units for commuter services entered service in 2003, and 45 Class 444 five-car units for long-distance inter-city services entered service in 2004. An additional order for a further 17 Class 450 units was placed, with the trains in service by late 2006 and early 2007.