Vincent St Claire, managing director of Arval PHH's fleet division, confirmed that take-up of diesel cars on contract hire had increased by up to 40%.
He believed that the widespread use of diesel could result in less fuel duty being paid at the pumps, encouraging an increase in duty on the fuel.
But Stewart Whyte, managing director of Fleet Audits, disagreed, saying: 'Despite increases in sales of diesel cars, the truck and bus sector is a powerful lobby and could affect any decision the Chancellor of the Exchequer might make on diesel duty.'
Alastair Kendrick, director of PAYE and NI solutions at Ernst & Young, said an alternative might be for diesel fuel duty to increase, while bus and HGV operators were offered increased rebates.
He added: 'I think that bus operators and HGV operators are fairly relaxed about diesel duty because they know any increase is unlikely to affect them, but company car fleets cannot expect the same benefits.'