British strikers protesting against the use of foreign workers are up against one of the basic principles enshrined in European Union law.
When the UK joined the then Common Market in 1973, the country accepted the right of nationals of any other member state automatically to live and work in Britain.
But at that time there were only eight other member states, all of them prosperous and unlikely to trigger mass migration to the UK to look for jobs.
In fact, in the strike-hit Britain of the seventies, it seemed far more likely that UK workers would take advantage of EU club membership to look for jobs on the continent.
But the balance tipped massively when in May 2004, the EU - by then already 15 member states - took in ten more countries at once.
Eight were poor central and eastern European former communist countries struggling with democracy and freedom after the collapse of the Berlin Wall in 1989.
The "big bang" enlargement was seen as a major triumph for the EU in reuniting east and western Europe - but the huge disparities in wealth and circumstance soon caused headaches.
Workers from Poland, Hungary and the Czech Republic suddenly found a world of work and living standards on their doorstep that they could never have dreamed of.
In anticipation, the European Commission allowed existing member states to delay the opening up of living and working rights to the newcomers for three years after 2004.
The aim was to reduce the economic and social shockwave of any possible stampede for relatively highly paid western jobs.
Only three countries - Britain, Ireland and Sweden - decided to apply full EU rights to the new member states from the start.
Some others now have, while yet more are waiting until the last possible moment - 2011 - to be obliged to apply full worker and residency rights to the newcomers.
For those who have granted full access, the result has been branded "Polish plumber syndrome", in which British and other employers in the "old" EU countries have been unable to resist the opportunity to snap up cheaper "foreign" labour.
The European Commission acknowledges the threat, but points out the benefits for western countries to set up shop in the "new" member states. And it has been updating other EU rules to help ensure a level playing field - EU-wide standards for workers' rights, social and welfare benefits and the mutual recognition of professional qualifications.
German and Austrian firms in particular are doing big business in Central and Eastern Europe - partly because they are closest and can re-locate most cheaply and easily.
At its best, the system is seen as a triumph for the EU founding fathers who in 1957 established free movement of people across Europe as one of the most important rights the post-war Common Market could offer citizens.
At its worst, it is seen as triggering a free-for-all as the poorest migrate to find the richest pickings, and employers in rich countries relocate to the cheapest parts of the Union.
Today Andy Smith, spokesman for the Campaign for an Independent Britain (CIB), said: "Already, opposition is growing in Britain to the policy of free migration within the EU, which allows people from Continental Europe to take jobs in the UK.
Campaigns against foreign ownership of key British companies and against overseas 'outsourcing' of manufacturing and services are gaining ground. It is hardly surprising that in the recent public opinion poll 71 per cent of voters said they wanted a national referendum to decide whether Britain remains in the EU, and 75 per cent said they thought UK politicians don't do enough to stand up for British interests in Europe."