How much more evidence do we need? Saturday’s Britain Needs a Pay Rise rally couldn’t have had a better endorsement than the alarming official figures that emerged this week. Low pay is not just unjust, it’s crippling the country’s finances.

On Wednesday Steve Machin, research director at the LSE’s centre for economic performance, laid out to a meeting of economists the collected evidence on the nature of falling pay – and warned that this is beginning to look not like a slow recovery in wages, but a permanent, structural feature of the UK economy. He showed how the group-think of economic forecasters has consistently and wildly over-estimated an expected increase in wages: the OBR forecast for March this year was a wage rise of 4.3%. What happened has been a continuing real fall.

“There has been a startling and unprecedented lack of wage growth as unemployment falls,” Machin says. The “herd mentality” of forecasters is always to expect things to improve, but there is no sign they are right. This begins to look like the new permanent, as flatlining real median pay began back in 2003, long before the crash. Nor, finds Machin, is immigration a cause of falling pay: areas with high or low immigration saw pay fall equally.

Median workers – the middle mark where half earn more and half earn less – have seen their wages fall by a real 10% since 2008 on the consumer price index (CPI) inflation measure – and by more using the retail price index (RPI) that includes housing costs.

Meanwhile, profits have held up well, but they aren’t shared with the workforce. Not for 140 years has there been a wage drop such as the one suffered in the past six years, “not since Victorian times”.

Is there a danger we are growing used to the idea, less shocked by most people seeing their pay fall year after year? But if the Treasury was unmoved by the TUC campaign, it should have been chilled by two other sets of figures this week showing how persistent low pay is draining its coffers dry.

If people don’t earn, they don’t pay tax – and Treasury receipts are falling way below forecast – needing higher tax or deeper cuts to fill the gap. The rising numbers in self-employment are among the non-taxpayers, not budding entrepreneurs but earning an average £10,000, mini-cab driver their most frequent occupation.

Another set of OBR figures this week should set the Treasury shuddering, showing that the benefit cap, trumpeted by George Osborne as his trap for Labour, will be burst. Low pay means many more poverty incomes have to be topped up with extra tax credits. Now add in the government’s disastrous housing policies – and three decades of successive governments’ failure to build council houses – and those pigeons have come home to roost in the Treasury.

As private renting has overtaken the number of people in social housing, the housing benefit bill has shot through the roof – uncontrollable despite Iain Duncan Smith’s draconian removals of the poor to ever lower rent districts. Unchecked house prices sending private rents soaring have burned a hole in the housing benefit budget as the low paid need their rent subsidised, with often sky-high rents for lousy properties.

Prof Machin has sounded a loud claxon. Unless there is a sharp change of policy, Britain may have left behind the postwar era of an average 2% pay rise a year. Instead we appear to be entering a darker time of permanent low pay where most people always fall further behind the small cadres of higher earners. And the Treasury – and the taxpayer – picks up an ever bigger bill for the wages employers refuse to pay.