A total of 91.3% of trains ran on time in the period July 21 to August 17, Network Rail (NR) said.
This compared with a figure of 92% in the same period last year.
The poorest late-summer punctuality was on the two main London to Scotland routes - East Coast and the Virgin-run West Coast.
Only 85.7% of East Coast and only 86.2% of West Coast trains were on time. Delays linked to NR accounted for 69% of the East Coast delays.
The Office of Rail Regulation (ORR) has warned NR about its punctuality record, particularly on the long-distance routes. Virgin Trains has also been particularly unhappy with NR's punctuality on West Coast.
Virgin's request to run extra services on the West Coast line was turned down and it has re-applied to start the services from next May.
An ORR spokesman said: "NR is not achieving the targets it was funded to deliver and is responsible for a large proportion of train delay. NR must quickly improve and catch up on planned renewal work to improve resilience, reliability and punctuality"
A spokesman for the Association of Train Operating Companies said: "More than nine out of 10 trains arrived as planned according to the Government-set measure for punctuality."
Manuel Cortes, leader of the TSSA rail union, said: "The only thing that runs on time on our railways is fare rises. They happen every 12 months despite how poor the service is that passengers have to suffer."