Japanese regulators have found inadequate fireproofing at more than one fifth of the nuclear reactors that went offline after the 2011 Fukushima crisis, a major daily said Tuesday.

The finding could delay their restart by several years in some cases, the Mainichi Shimbun said.

More than 10 of Japan's 50 reactors, excluding those at Fukushima, have flaws in fireproofing, the paper quoted sources at the industry ministry and the Nuclear Regulation Authority (NRA) as saying.

The Mainichi said these include the use of combustible electrical cables and equipment and machines sited too close together, potentially allowing a fire to spread even though the equipment is indispensable for maintaining safety.

The industry ministry has been investigating the issue while the NRA will soon conduct hearings with power companies, it said.

At some reactors the work to replace cables and renovate facilities could mean a delay of several years in restarting them, it quoted industry ministry sources as saying.

If renovation proves to be too costly, the reactors could be decommissioned, the sources said.

All but two of the nation's reactors remain offline after they were shut for safety checks in the wake of the March 2011 Fukushima meltdowns, sparked by a quake and tsunami.

The pro-business Liberal Democratic Party-led government said last week it would give the green light to restarting any reactors deemed safe by regulators.

New prime minister Shinzo Abe has also voiced his willingness to build new reactors.