Infrastructure problems on a main London to Scotland rail route contributed to an overall dip in nationwide train punctuality last month.
The difficulties, which included overhead line and track problems, meant Virgin Trains was able to run only 82.9% of services on time on its West Coast route in the period April 29 to May 26.
Overall, train companies ran 92.3% of trains on time during this period compared with 93.7% in the same period last year, rail infrastructure company Network Rail (NR) said.
The Office of Rail Regulation (ORR) has already warned NR to improve punctuality on long-distance services, including the West Coast line.
Today, Virgin Trains said the infrastructure problems on the West Coast line - which are the responsibility of NR - were "unacceptable" and that the ORR's action "does not deal adequately with the immediate issues".
Virgin went on: "We are concerned with the lack of focus on immediate improvement, as we and our customers must see short-term progress, especially as the Olympics and Paralympics will be a showcase for the industry."
Virgin's performance last month was also affected by temporary speed restrictions, a fatality and cable theft. In the same period last year, Virgin was able to run nearly 90% of trains on time.
The best-performing train company last month was London to Tilbury and Southend operator c2c which ran 97.2% of trains on time.
Apart from Virgin, the poorest-performing company was the other main London to Scotland operator, East Coast, which ran 87.6% of trains on time.