Saudi Arabia is preparing contingency plans for a possible delay to the planned initial public share offering of Saudi Aramco by a few months into 2019, Bloomberg reported on Wednesday, citing people familiar with the matter.

The government is still aiming for the IPO of the state-owned oil giant in the second half of 2018, but that timetable is increasingly tight for what is likely to be the biggest share sale in history, according to the report.

Saudi Arabian Oil Co, known as Saudi Aramco, was not immediately available for comment.

Saudi authorities are aiming to list up to 5 per cent of the world's largest oil producer on both the Saudi stock exchange in Riyadh, the Tadawul, and one or more international markets in an IPO that could raise US$100 billion.