It took the top slot from the Ford Mondeo, which moved to second place on the Lex list, while the Volkswagen Golf moved down to third place.
The study also found that diesel remained popular with Lex drivers, accounting for 50% of all orders.
Out of the 35,000 vehicastrales Lex purchased for its customers during last year, Ford and Vauxhall retained their dominance, taking five out of the top 10 positions.
The company said that when comparing the list with 2001, there were gradual signs of company motorists downsizing their cars in a bid to reduce their benefit-in-kind tax bills.
It added: 'Proof of this theory is that the VW Passat has gone from fifth most popular car in 2001 to ninth in 2003 while the BMW 3-series has also gone from second place in 2001 to fifth place.'
'With more drivers choosing their company car with their wallet rather than their heart following the introduction of the CO2 emission-based tax system in April, we are buying more smaller cars for our customers,' said Steve Jones, Lex Vehicle Leasing pricing manager.
He added: 'The Focus and Astra have both benefited from this trend, but the biggest change is the number of diesel cars we are now buying. Diesel penetration of the fleet has grown by 25% over the past three years, but we believe it will stabilise over the next two years.'
|Lex Fleet 2003 Top 10|
|Vauxhall Corsa:||8th||9th||(-)||VW Passat:||9th||8th||5th|