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Immigration is one of the most sensitive issues in British politics. Polls indicate that it's a major concern to many people, but it's an issue which politicians in the three main parties - and indeed many of us in the media - have been reluctant to discuss much until quite recently.

Because of the economic crisis from which the UK and the rest of the EU is only slowly emerging and the prospect of unemployment remaining quite high for some time, concern about immigration seems likely to remain a hot topic.

So The World Tonight has got together with the leading think tank to host a special debate on the economic, social and cultural costs and benefits of immigration to the UK.

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In a programme to be broadcast on Friday, presenter Robin Lustig will chair the debate between , head of the Equality and Human Rights Commission; , the director-general of the British Chambers of Commerce; , Baroness Flather, who was the first Asian woman to receive a peerage; and , director of the think tank the Centre for Social Cohesion.

Any discussion of immigration is fraught with difficulties around definitions.

The exact number of people coming in and out of the UK and how many stay for any length of time is often disputed, because it is difficult to count the exact number of people entering and leaving the country and for how long they stay in or out. This means the interpretation of official statistics is argued over - for instance, by .

Who exactly constitutes an immigrant? The UN defines it as someone who moves to another country and stays more than a year, but how many of these people stay for more than a few years before going home or moving on to another country, and how many settle in the UK permanently?

There is often confusion between migrants and refugees. The latter are people who literally seek refuge from persecution in their own countries and which the UK is bound by to host if they can prove they have "a well-founded fear of being persecuted for reasons of race, religion, nationality, membership of a particular social group or political opinion".

What powers does the government have to control immigration? About a quarter of immigrants come from other EU countries; because of the freedom of movement within the European Union, short of leaving the EU, this cannot be stopped - although their right to work can be restricted for a transitional period for new members, something the UK chose not to do in the case of Poland but did in the case of Romania and Bulgaria. It also has to be remembered of course that many British citizens exercise this right to live in France or Spain, for instance.

So when we talk about controlling immigration, we are talking about migrants coming from countries outside the EU, which last year was just over half the total of immigrants, and it is those numbers the .

Then there is the whole question of illegal immigration; by its nature, this is not counted and is more difficult to assess the level of and to control.

As the world becomes more interconnected and globalised, both economically and culturally, it is difficult to imagine that immigration can be reduced dramatically. But it is also important to note that the absolute numbers of people moving in and out of most countries is relatively small, if you exclude major refugee movements because of war or natural disaster.

So in our debate we hope to establish clear parameters for our discussion and go on to have a debate on the concerns people have and what the best approach to immigration should be - take a listen and let us know what you think.

Alistair Burnett is the editor of The World Tonight.