British postal workers began what could become the longest strike in the Post Office's 300-year history Monday as part of a wave of industrial action that is also threatening Christmas travel chaos.
Thousands of members of the Communication Workers Union (CWU) are planning to strike for five days in protest at job losses and pension changes, upping the pressure on Conservative Prime Minister Theresa May.
The CWU said around 3,500 workers would be striking, hitting hundreds of post offices at the busiest time of the year for Britain's postal service.
A further 1,000 Royal Mail delivery staff could take unofficial industrial action by refusing to cross the picket lines at post offices although Royal Mail said it was expecting "little or no impact" from strikes.
Members of the Rail, Maritime and Transport (RMT) union at Southern Railway have also said they will strike on Monday and Tuesday in a long-running labour dispute that has affected 300,000 passengers for months.
British Airways cabin crew are also holding talks on Monday aimed at averting strikes on Christmas Day and Boxing Day, while Swissport baggage handlers are planning walkouts on Friday and Saturday.
May is facing calls from members of her own Conservative Party to intervene and tighten legislation to prevent rail strikes.
"Any change to union laws would not be able to solve the current disputes," a Downing Street source was quoted by the Guardian newspaper as saying.
"They will be solved by mediation and we would urge the unions to get round the table."
But Chris Philp, a Conservative MP, told BBC radio: "Unions have pushed this too far".
"We need fresh union legislation," he said.
Frances O'Grady, general secretary of the Trades Union Congress, said workers were still suffering from the global financial crisis and there should be more attention on "what's wrong with these companies".
The political row heated up after the Sunday Times published a video showing RMT union president Sean Hoyle telling a rally that trade unions were working together to "bring down" the government.
Industrial action has risen sharply in Britain this year, against the background of a sharp leftward turn in the opposition Labour party under its veteran Socialist leader Jeremy Corbyn.
The number of days lost to strike action in the year to October 31 totalled 304,000 compared to 205,000 days for the preceding 12-month period, according to the Office for National Statistics.
Tom Kerr Williams, employment law and industrial relations expert at consultants PWC, told City A.M. the common factor in the strikes was that workers "feel uncertain as a result of economic pressure".