Britain faces biggest rail strike in over 30 years
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RMT general-secretary Mick Lynch described as "unacceptable" offers of below-inflation pay rises by both overground train operators and London Underground that runs the Tube in the capital.
Transport Secretary Grant Shapps said the government was doing everything it could to minimise the expected "mass disruption".
But he told parliament on Monday: "It's estimated that around 20 percent of planned services will operate, focused on key workers, main population centres and critical freight routes."
The strikes -- also on Thursday and Saturday -- risk causing significant disruption to major events including the Glastonbury music festival.
Schools are warning that thousands of teenagers taking national exams will also be affected.
RMT members on the London Underground are additionally staging a 24-hour Tube train stoppage Tuesday.
The union argues the strikes are necessary as wages have failed to keep pace with UK inflation, which has hit a 40-year high and is on course to keep rising.
- Teachers, lawyers, NHS -
Countries around the world are being hit by decades-high inflation as the Ukraine war and the easing of Covid restrictions fuel energy and food price hikes.
Unions warn also that railway jobs are at risk, with passenger traffic yet to fully recover after the lifting of coronavirus pandemic lockdowns.
The strikes are compounding wider travel chaos after airlines were forced to cut flights owing to staff shortages, causing long delays and frustration for passengers.
Thousands of workers were sacked in the aviation industry during the pandemic but the sector is now struggling to recruit workers as travel demand rebounds following the lifting of lockdowns.
Other areas of the public sector are meanwhile set to hold strikes.
The Criminal Bar Association, representing senior lawyers in England and Wales, have voted to strike from next week in a row over legal aid funding.
Justice minister James Cartlidge called the walk-out "disappointing" given that the court system is already battling significant backlogs in cases caused by the pandemic.
Four weeks of action begin on Monday and Tuesday, increasing by one day each week until a five-day strike from July 18.
Teaching staff and workers in the state-run National Health Service are reportedly also mulling strike action.
And several other transport unions are balloting members over possible stoppages that could occur in the coming weeks.