The Japanese government is likely to postpone further plans to extend a bullet train line to Sapporo, capital of the northern Japanese island of Hokkaido, by the spring of 2031.

The project is intended to extend the existing line, which has its northern terminus at the southern Hokkaido town of Hakuto.

Around 80% of the extension will be underground, and when complete will cut the time taken to travel between Sapporo and the south coast city of Hakodate from 3.15 hours to one. When complete, the whole line will be 212km long.

Government sources told local media that the delay was caused by the discovery of an unexpected rock formation where tunnelling was planned, as well as expected labour shortages caused in part by overtime regulations due to come into force next year.

The cost of the line, put at $11.2bn when it was approved in 2012, has since risen to around $15.5bn.

The delay, if it is announced, will follow Sapporo’s decision to drop its bid to host the 2030 Winter Olympics, the original deadline for completion. The project was, in any case, due to miss that target unless work was accelerated.

The city is expected to enter the running for the 2034 games, by which time it should be connected to the Japanese national shinkansen network.

The transport ministry and rail operator JR Hokkaido will now arrange a new deadline.