Should immigration rules be relaxed on economic grounds to enable the UK to find more workers to plug shortages in key industries and to support the ageing population?

It is a question politicians have to face as they battle with illegal immigration on one side and recruiting staff such as teachers and nurses on the other.

Environment editor of The Times Anthony Browne argues against large-scale immigration but Professor Nigel Harris of University College London disagrees and says all immigration controls should be abolished.

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Anthony Browne

I'm not anti-immigration, I'm in favour of balance and sustainable immigration and what we have in Britain is completely unbalanced.

About 500,000 people arrived in the UK last year and 300,000 left, so we had net positive immigration of 200,000 and that's quadrupling the rate of our population growth.

So our population is now escalating at its fastest rate since the 1960s and there are all sorts of consequences of that in terms of house building programmes, environmental degradation and so on.

If you bring in young workers to look after old workers, then what do you do when the young workers retire?

Anthony Browne, Environment editor, The Times

We don't have a declining population.

There are more births each year than deaths and the declining workforce is expected to increase in the next 20 years, partly because women's retirement age has risen from 60 to 65.

If you bring in young workers to look after old workers, then what do you do when the young workers retire?

You have to bring more in and you end up with a perpetually growing population and to do that, the United Nations estimates you would have to have 130 million people in Britain by 2050.

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Professor Nigel Harris

I want to move towards a world in which people are free to migrate to come and to go as they want and to work where they want with complete open borders for all countries.

Calculations suggest that the world gross domestic product could be increased three times over with free immigration, that is, the workforce responding to the location of jobs.

Already, the aged and disabled depend upon immigrant workers and that's going to get steadily more extreme

Professor Nigel Harris, University College London

If you then look at this country, the arguments against immigration seem to be unsound, but more important than that is the ageing of the population and therefore its increasing dependence on labour intensive services.

Already, the aged and disabled depend upon immigrant workers and that's going to get steadily more extreme.

The argument is that the jobs done by immigrants are not done by the natives.

If we raise the wages of those jobs, will the natives work there? Will they man the caring professions?

I think all the evidence suggests the scale of costs here will make those services unaffordable for the bottom quarter of the aged population, so we are going to have to face the question of free movement of, in this case, unskilled labour.

Nobody I know except me is arguing for that.