It was scheduled to have been re-opened this Thursday but this has been put off after fresh reports of Tamil Tiger threats against the port, officials said.
"Recently, we got information about a possible attack," a senior official said. "If not we had planned to open the northern entrance from January 1."
The Tigers staged an abortive attack on the port using a speed boat and divers who came in through the northern entrance in the 1990s.
The entrance has been kept close most of the time since then despite repeated promises by the authorities to re-open it.
The channel had got silted during this time, hence the need for dredging.
The closure of the entrance channel has long been an issue with port users who complain it causes delays to shipping which are confined to the use of a single entrance channel where only one ship can pass at a time.
Port users have said such navigation restrictions cause congestion in the harbour, resulting in delays in cargo handling and turning around container ships.
The SLPA has tested sonar systems to guard the harbour entrances and had planned to re-open the northern entrance when these systems were fully in place, officials said.
"Once re-opened, the SLPA is aiming to keep the northern entrance open at least during day time," the official said.
The Tiger rebels recently threatened to attack economic targets as security forces closed in on their last remaining strongholds in the island's north-eastern region.