Under the cap, no more than 100,000 travelers per day will be allowed to depart from the airport until October 29.
This capacity limitation was initially only meant to last until September 11, but has now been postponed to allow for more reliable passenger service.
The move comes after an improvement in punctuality and fewer last-minute cancellations following the introduction of a temporary cap in July.
Ross Baker, Heathrow’s Chief Commercial Officer, said: “Our primary concern is to ensure that we offer our passengers a reliable service when they travel.
“That’s why we introduced temporary capacity restrictions in July, which have already improved travel during the summer holidays.
“We want to lift the cap as soon as possible, but we can only do so when we are confident that everyone working at the airport has the resources to provide the service our passengers deserve.”
Capacity limits “under review”
Passengers flying to and from Britain’s busiest airport have faced severe disruption in recent months, with long security queues and baggage system failures.
However, a temporary cap imposed by Heathrow in July resulted in “fewer last-minute cancellations” and “shorter waiting times for bags”, according to the airport.
Heathrow said the capacity limits would continue to be reviewed and “could be lifted sooner if a sustainable picture of better resilience and a significant increase in resource levels emerges”.
But consumer rights group What? has warned the move will leave the plans of thousands of passengers in limbo.
Guy Hobbs, the organization’s travel editor, described the situation as “chaotic” and urged the airport to provide holidaymakers with clarity on the flight schedule.
He said: “Although the passenger cap extension may help Heathrow prevent a repeat of the unacceptable last-minute cancellations we saw earlier in the summer, thousands of people will now be concerned that their travel plans could be dashed.
“Heathrow and affected airlines must act promptly to provide travelers with clarity as to which flights are being cut, and airlines must ensure affected passengers are aware of their rights to rebooking or refunds.
“The airline industry and government must ensure that this mess is resolved as soon as possible – passenger caps must not continue indefinitely.”
If flights are cancelled, airlines must offer to book passengers on an alternative route as close as possible to the original arrival time.