In proposing that temporary and agency workers should benefit from the same pay and basic conditions as their fully accredited counterparts, Farrelly's bill drills into an issue that barely intrudes on the political mainstream: the casualisation of thousands of workplaces, and the alleged slow death of the Real Job. Around 1.4 million people currently work in the temporary and agency sector, millions more feel its downward pull on their working lives - and at its current rate of growth, millions more soon will do. Unfortunately, the involvement of the trade unions serves to confirm that the issue lies as far from middle England as can be, and you thus arrive at yet another illustration of how contorted Westminster politics has become: the political class blithely yakking about "rising aspirations", while millions of people's hopes are plummeting at speed.

At the core of all this is a red-hot bundle of concerns around immigration, so those lobbying for change choose their words very carefully. No one is exactly sure how many migrant workers are employed in temporary and agency work - though foreign-born people increasingly form the sector's bedrock, and dominate its role in such areas as catering, private security and construction. This week's government report on immigration's economic impact might have told a rosy story, though one could just about discern these developments in some of its more unsettling passages - admissions that workers from the A8 and A2 countries (those eastern European states who joined the EU in 2004, as well as Romania and Bulgaria) "earn noticeably less than UK-born workers", and that migrant labour has effected a "modest" downward pull on wages at the economy's bottom end. Casualised work hovers behind both those points - though bizarrely, the report contains no mention of temporary and agency employment, nor any data about how migrants fare when it comes to holidays, pensions, sick pay and the like.

On the ground, however, alarming evidence is mounting up. A few months ago I spoke to a manufacturing employee from the West Midlands who works in a factory producing car parts. Three years ago, his bosses began the mass recruitment of a new kind of worker. A dwindling number of long-standing staff were on £11 an hour; the new arrivals - many of whom barely knew what they were doing - worked 12-hour days for £4 an hour less, had none of the usual entitlements to paid holidays or sick leave, and were seemingly arriving and leaving through a revolving door. Within 18 months, for every "core" worker, there were two supplied by agencies, many of whom were from Poland, Cameroon or Senegal. The walls were quickly smattered with racist graffiti and the level of scrap increased fast. Under union pressure, the company relented and proposed a scheme whereby long-standing agency workers could eventually join the accredited workforce, and, in its wake, the rancorous atmosphere began to improve.

Set against developments that are defining an ever-increasing share of the economy, these people were lucky. Trade unionists cite no end of altogether bleaker case studies: three-tier workplaces in which indigenous British employees sit precariously at the top, flimsily employed Poles come further down, and thoroughly casualised Hungarians and Slovakians are left right at the bottom; increasing numbers of people whose lack of sick pay forces them into working while ill. On the stories go: meat-processing workers in Monmouthshire threatened with redundancy unless they downgraded to agency terms, and then fired; agency street cleaners in Salford who must show up at dawn each day to see if they're required for work. The dereliction of formal employment applies to both private and public sectors, and the issue spreads way beyond the workplace: there is increasing evidence, for example, that casualisation is feeding into the burgeoning sub-prime mortgage market.

Seemingly frit when it comes to dealing with the economic nitty-gritty, the government's absence of answers is crystallised by that ugly and pretty much meaningless slogan "British jobs for British workers". Being on the centre-left should surely be less about appropriating the vernacular of the ultra-right than understanding the need for action at the sharp end: as Blair's old formulation would have it, being tough on social discord and tough on the causes of social discord. But no, give or take recent talk about increased enforcement of the minimum wage and timid plans for regulation that will swerve past the essential issues of unequal pay, the poisonous status quo remains.

When the election finally arrives, beware. Given its talent for issuing leaflets that read more like Socialist Worker than Mein Kampf, the British National Party is making hay with the issue of casual labour, as I was recently reminded while reading The Triumph of the Political Class, a new book by the Mail columnist Peter Oborne. An elegant tirade against a cross-party cabal either in thrall to vested interests or so lost in the woods of electoral arithmetic that the stuff of real lives scares them, one of its most sobering sections deals with the rise of the BNP "in Barking, Dagenham, Dewsbury, Leeds and Burnley" and its place in what he sees as an "insurgency against the political class".

"The estrangement between a tiny governing elite and mainstream British society is one of the overwhelming themes of our age, and will only get more desperate, and more dangerous," he says. At the top, an uneasy silence about one of our most urgent issues proves the point, but who's listening?

john.harris@theguardian.com