India, June 3 -- Delhi airport's third runway, which has been shut since mid-February, may reopen only in September, several weeks after originally planned, as authorities work to shore up the tarmac's aircraft-handling capacity by 10%, said officials aware of the matter.

The runway was shut for long-overdue strengthening and upgrades. It was due to reopen in early July, but this date has been pushed back as authorities put in place key systems to add at least eight flights an hour, parallel with the earlier work, to curb the possibility of another shutdown in the short term, said four officials aware of the developments.

One of the officials, asking not to be named, said: "Considering safety and operational benefits, the proposal has a...