But Vauxhall described the situation as a 'temporary blip' caused by the run-out of the old model.
It had led the way after beating fierce rival Ford to the top of the fleet sales table for the first time last year.
But latest figures show Ford's fleet sales overtook Vauxhall's during the month of April – although the Luton-based manufacturer is still on top for year-to-date sales.
The company said the expected drop in sales of its previous Astra was to blame. The new model went on sale this month.
Vauxhall fleet sales director Maurice Howkins said: 'We may have seen a temporary blip in the fleet sales figures due to run-out of the existing Astra but are still ahead of the competition on the year-to-date figures.
'And with overflowing order banks for new Astra on both fleet and retail, we are expecting a very strong sales performance in the coming months.'
Year-to-date figures show mixed fortunes for manufacturers in terms of fleet sales. Five manufacturers increased their sales when compared to the same period last year – Ford (10%), Vauxhall (4.3%), Renault (13.3%), Toyota (23.3%) and Honda (26%).
Volkswagen, Peugeot, Nissan, Citroen and Audi also suffered drops, with Citroen having the biggest percentage fall of 20.7%.
Earlier this year, the company revealed plans to slash the manufacturer's fleet business by 9,000 units this year.
The group plans to pull 6,000 units out of the daily rental market and 3,000 units from Motability, the UK's largest fleet. However, Citroen says it will continue dealing with fleets that generate profits (Fleet News, April 8).
As reported on Fleet NewsNet last week, the motor industry is heading for record sales this year.
Sales to sub-25 fleets continues to perform well and were up for the eleventh successive month, with a 10.6% rise.
Total sales, including private registrations, dipped 3.8% to 187,004 units but year-to-date sales were up by 3.9% to 943,010 units.