Only around a fifth of Britain’s train services will operate on Thursday and Saturday during the latest rail strikes.
Network Rail urged passengers to “only travel by train if absolutely necessary” as it confirmed just 20% of services will run and only half of lines will be open.
Services will start later and finish much earlier than usual, with trains running between 7.30am and 6.30pm.
Many parts of Britain will have no trains, including most of Scotland and Wales.
There will also be disruption during the following days, with service levels reaching 70% on Friday and 85% on Sunday.
Thousands of members of the Rail, Maritime and Transport union (RMT), Transport Salaried Staffs’ Association (TSSA) and Unite at Network Rail and 14 train companies will strike in the long-running row over pay, jobs and conditions.
There will be around 200 more services on each strike day compared with previous walkouts by members of the RMT as the school summer holiday has ended in Scotland, meaning more contingency staff are available.
Network Rail chief executive Andrew Haines said: “It saddens me that we are again having to ask passengers to stay away from the railway for two days this week due to unnecessary strike action, when we should be helping them enjoy their summers.
“We have made a good and fair offer but, with the exception of our TSSA management grades who accepted the deal, our unions are refusing to let our employees have a say, and sadly that means more disruption on the rail network.
“We’ll run as many services as we can on Thursday and Saturday, but it will only be around a fifth of the usual timetable, so please only travel if absolutely necessary and if you must travel, plan ahead and check when your last train will be.”
Steve Montgomery, who chairs the Rail Delivery Group, said: “The leadership of RMT and TSSA are imposing yet more uncertainty on passengers and businesses by disrupting passengers’ travel plans.
“While we will do all that we can to minimise the impact and to get people where they need to be, passengers should only travel by rail if absolutely necessary and be aware that services may start later the morning after strikes.
“If you’re not able to travel on 18 or 20 August, you can use your ticket either the day before or up to and including 23 August, otherwise you will be able to change your ticket or claim a refund.
“The railway is vital to this country, but with passenger revenue still 20% below 2019 levels, securing a strong future means we have to change and move with the times.
“Only then can we fund the pay rise we want to give our people, while delivering the more reliable Sunday services and improved punctuality our passengers deserve.”
There is also a strike by London Underground workers on Friday which is expected to cause severe disruption.