About 2,000 workers at 17 sites across the country have been taking part in unofficial strikes protesting at the use of foreign labour. The protests started at an oil refinery in Lincolnshire. An Italian company that has been subcontracted to work on the site is using its existing Italian and Portuguese employees. Trade unions maintain that the jobs ought to have gone to British workers. Such demands echo a worrying global trend to economic nationalism.

The strikers' placards invoke a slogan coined by Gordon Brown in 2007: “British jobs for British workers.” Despite much criticism then and since, Mr Brown insists that he has no regrets about using those words.

That response is either disingenuous or obtuse, and is in either case discreditable. The UK is bound through membership of the European Union to welcome workers from other EU states. But this country's treaty obligations are the least of the objections to the Prime Minister's formulation and the wildcat strikes. British workers who are unable to find jobs in a severe downturn have understandable reason for frustration; but it is economically illiterate to suppose that domestic living standards and employment are damaged by the free movement of labour.

Economists refer to the “lump of labour fallacy”. This is the notion that there is a fixed amount of work to be done in the world, so that if jobs are taken by foreign workers then domestic workers will lose out. A similar grievously mistaken notion was advanced, and refuted, when women entered the labour force in large numbers for the first time. Workers not only take jobs, they also create jobs. When they spend their wages, they increase the demand for consumer goods and services. Even large-scale immigration has only a minuscule effect on unemployment and wage levels.

Mr Brown knows this. Yesterday he pointed once more to the dangers of protectionism. Yet he has been careless about the connotations of his “British workers” message. When this newspaper observed that the Prime Minister's slogan was echoed in the propaganda of the British National Party, Downing Street responded extraordinarily that, in fact, Mr Brown had used the slogan first - as if this somehow softened rather than amply confirmed its inflammatory content.

Unfortunately, the protests against foreign labour are part of a populist impulse that often asserts itself in economic downturns. The issue is of particular relevance in the US, where the fiscal stimulus package making its way through Congress includes a “buy America” clause that would bar foreign iron and steel producers from infrastructure projects. Erecting barriers to imports and to labour is a tempting route, and a disastrous one. President Obama has yet to retreat from the ferociously irresponsible protectionist rhetoric deployed in his primary campaigns. It is scarcely conceivable that his Treasury appointments are comfortable with this position, especially as they know economic history well.

The Great Depression is the precedent that today's policymakers fear. Its human costs were measured in unemployment, homelessness and hunger. It was aggravated by protectionism. With the Smoot-Hawley Tariff Act in the United States, the adoption of tariffs and imperial preference by Britain and the division of the global economy into currency blocs, world trade by the end of 1932 was barely one third of the level of 1929. The World Bank now forecasts that world trade will contract in 2009, for the first time since 1982. A renewed Doha Round of trade talks is needed.

In these circumstances, policymakers have an onerous responsibility but an easy message to convey. Economic nationalism is morally objectionable; it is also a fast track to penury.