The reactor, operating since 1969, was shut down in March 2015 after defects were found in its pressure vessel.
But in November its owner, energy company Axpo, said it had completed tests required by the Federal Nuclear Safety Inspectorate (ENSI) and found that the reactor was safe to be restarted.
The tests were carried out on a replica of the reactor pressure vessel.
Axpo submitted its test results to ENSI for review, saying it had “no safety concerns” about the plant and that it could operate until 2030.
But on Tuesday Swiss media reported that ENSI's examination of the test results was taking longer than previously assumed. The inspectorate has requested further information from Axpo and will not be taking a decision on restarting the plant until March 2017, said Tages Anzeiger.
According to the paper, putting Beznau I out of service has cost Axpo 200 million francs to date, a figure that will probably rise with this extension.
In November the Swiss public voted against an initiative led by the Green Party that could have seen Beznau I shut down permanently as soon as next year.
The initiative aimed to withdraw the country from nuclear power much earlier than the Swiss government planned.
Beznau I sits on an artificial island on the Aare river, 37 kilometres from central Zurich and ten kilometres from the German border.
The second reactor on the site was built in 1971.