It is the subway's first system-wide closure for non-weather-related conditions in 40 years of operations Metro chairman Jack Evans said Tuesday admitting it would likely trigger transport misery for hundreds of thousands of commuters and tourists.

The metro system -- the second busiest in the United States after New York -- serves about 700000 customers a day in Washington and the neighboring states of Maryland and Virginia where many commute from into the capital.

The closure which was expected to spark fierce competition for taxis and see buses badly overloaded was due to begin at midnight (0400 GMT Wednesday).

"We will suspend rail service for 24 hours to inspect 600 underground jumper cables. The entire rail system will be closed until 5:00 am Thursday" Paul Wiedefeld general manager of the Washington Metropolitan Area Transit Authority told a news conference.

It is the latest bad publicity for the Washington Metro which has 91 stations and began operating in 1976.

In January last year a woman died and dozens of people were injured after thick smoke filled a metro tunnel during evening rush hour due to an electrical malfunction.

The whole system -- along with all buses -- also closed for an entire weekend during the "Snowzilla" blizzard that bombarded much of the US East Coast in January.

Then on Monday this week a cable fire erupted in a tunnel sparking major delays on three lines -- and ultimately the decision to shut the system down.

An investigation was still under way Wiedefeld said but "commonalities" were found between Monday's fire and the deadly January 2015 incident.

"While the risk to the public is very low I cannot rule out a potential life-safety issue here and this is why we must take this action immediately" he said.

"When I say safety is our highest priority I mean it. That sometimes means making tough unpopular decisions and this is one of those for sure.

"I fully recognize the hardship this causes to the region and to the community."

- 'Safety is paramount' -

Many in the US capital rely on the Metro which has six color-coded rail lines to get to work or visit tourist sites.

But residents frequently complain about delays long waits station closures and weekend disruptions caused by scheduled track work.

Evans admitted that the decision would cause "a lot of congestion."

"It's going to be very difficult we recognize for people to get in to work for school children to get to school for everyone who relies on Metro" he said.

"But safety is paramount and we need to get to the bottom of this."

Government offices will remain open Wednesday but employees can take unscheduled leave or work from home while schools also will keep their doors open.

"We are working with Metro to add additional bus service" officials said on Twitter adding that late arrivals and absences would be excused.

The city's public bike-sharing service waived membership fees for Wednesday to help alleviate what was expected to be a crush on buses although rides over 30 minutes long would still cost.