Drivethedeal.com, which sells new and used cars online, sources cars from UK franchised main dealers and says it has supplied cars to fleets ranging in size from three to 600 vehicles, two of which are public limited companies.
Research into who had bought or enquired about cars on the website this year has shown an increasing number coming from fleet operators.
Drivethedeal managing director Richard Sanders said: 'The two PLCs we supply have 180 and 600 cars respectively and enjoy being able to come to a single source for all their car requirements.
'They like the fact that we have already done the negotiating work to achieve generous discounts on all models and they can deal with one account manager, rather than 20 separate dealer sales managers.
'The smaller fleets - which normally do not have a structured car policy - like the fact they are benefiting from our bulk-buying capabilities and through a single call, or five minutes on our website they can research the best discounts and order their car through a single contact.'
He added that many buyers were also employees using their company car cash allowance.