The action taken by the ASLEF and RMT unions comes after drivers in the two unions decided to shun a 15% pay rise offer.
This means Metro train crew are not working any additional paid voluntary shifts.
Nexus says runs Metro as a public service without profit – all the money it receives in fares go towards the cost of operating, including employee wages.
It says it cannot afford to go beyond the offer it has made.
In a statement, Nexus – the operator of the Tyne and Wear Metro – said: “This is not the same as a strike, which would see train crew refusing to work at all, because most will still be coming in to complete rostered shifts.
"It will mean that some trains will be cancelled because there will be no train crew to operate them.
“There will be less frequent services and busier trains, but Metro will still be running to all destinations.
“We will forecast the level of service we anticipate being able to run each day and publish it in advance the help passengers to plan.”
Here’s what you need to know ahead of your journey:
There may be less frequent Metro services and busier trains on some days, but Metro will still be running to all destinations. Nexus aims to protect services in the peak ‘rush hour’ periods, when passengers rely on them most, as much as possible. You may have to wait longer for the next train, so please leave extra time for your journey. There may be occasions when trains are cancelled at short notice. Each day’s Metro service level will be graded during the industrial action: Green - fair service where some trains maybe cancelled; Amber - there may be some gaps in service and trains will be busier; Red – there will be a poor service and trains will be very busy with gaps between services.
This is how Metro services look set to be affected so far this week (service level subject to change):
Monday, December 2 : Green – fair service.
Tuesday, December 3: Amber – gaps in service.
Wednesday, December 4: Green – fair service.
Thursday, December 5: Amber – gaps in service.
Friday, December 6: Red – poor service.
Saturday, December 7: Red – poor service.
Passengers can keep up to date at nexus.org.uk and by following our daily updates on the Metro Twitter account: @My_Metro.
Each day it will publish details of the following day’s expected service level on www.nexus.org.uk