A pamphlet by the group suggests that Britain may have reached a "tipping point" beyond which it could no longer be seen as a single nation.
Shadow home secretary David Davis has called on the government to put a cap on those coming to the UK.
The Home Office said it had already announced a tough new points system aimed at immigrants.
The Civitas pamphlet - A Nation of Immigrants? - said the "seemingly reckless pace and scale" of immigration was bound to cause concern for people who saw the UK as a model of tolerance and freedom.
The 100-page booklet said Britain may have already reached a tipping point beyond which it could not longer be said to be a single nation.
"Once such a point is reached, political disintegration may be predicted to be not long in following," the report said.
Shadow home secretary David Davis said: "We know that unchecked immigration is putting pressure on housing and local services.
"Now this report shows that its effects are potentially even more serious."
"Given the limited number of schools, hospitals and houses, the government must apply a limit on the amount of people entering the country," he concluded.
A Home Office spokesman said the government supported legal migration which greatly benefited the economy and meant skilled migrants could fill labour gaps.
Ministers had also announced a new tough Australian-style points system for immigrants, he added.
"However, there are legitimate concerns about managing some of the effects of migration on communities. The government is listening to these concerns."
The spokesman also pointed to plans to create a panel to advise on where migrants would be best placed to fill gaps in the labour market.
David Conway, author of the Civitas report, said: "The view that Britain is a nation of immigrants suggests Britain has always experienced immigration on its present-day scale from time immemorial, which is by no means the case."        

In A Nation of Immigrants? David Conway takes issue with those who minimise the threat posed by mass immigration by claiming that this is nothing new; that we are a 'mongrel nation'; and that, in the words of the Commission on Racial Equality, 'everyone who lives in Britain today is either an immigrant or the descendant of an immigrant' (pp.2-3). He argues, to the contrary, that from the time England can be considered to have become a nation, immigration has never risen above very low levels and had no serious demographic impact until the last part of the twentieth century. Since 1997, however, Tony Blair's Labour government has effectively abandoned even the goal of limiting immigration. As a result, by encouraging unending mass immigration as a permanent feature of the political landscape, there may result a disintegration of the bonds that hold together the group of people that constitutes a nation:

'The country may possibly have already reached a tipping point beyond which it can no longer be said to contain a single nation. Should that point have been reached, then ironically, in the course of Britain having become a nation of immigrants, it would have ceased to be a nation. Once such a point is reached, political disintegration may be predicted to be not long in following'. (p.95)

David Conway shows just how small these famous historic waves of immigration actually were. French Protestants fleeing religious persecution, known as Huguenots, began arriving in Britain in the sixteenth century, and came in much larger numbers after the revocation of the Edict of Nantes in 1685. They settled initially in the East End of London and became successful entrepreneurs, especially in the silk industry. However, their overall numbers cannot have exceeded 50,000, representing about one per cent of the population (p.50).

The wave of Jews escaping the pogroms who began to arrive in London towards the end of the nineteenth century represented an even smaller percentage increase to the population. In the last quarter of the nineteenth century there were 155,811 Jewish immigrants(p.59), and even if we include immigration between the two world wars, their numbers would not have been much over 225,000 - representing about 0.5% of the population.

The situation changed significantly at the end of World War II, when Britain experienced large-scale immigration from New Commonwealth countries, especially in Asia and the West Indies. This led to a series of acts of parliament to restrict immigration, including the 1962 Commonwealth Immigrants Act (p.73) and the 1971 Immigration Act (p.76), which brought primary immigration from New Commonwealth countries under control. However, the political turbulence of the 1990s saw a great increase in applications for asylum, from about 4,000 a year in the 1980s to about 98,000 in 2000 (p.93). Numbers rose rapidly following the election of New Labour in 1997, and, in the face of great public disquiet, the government introduced measures to reduce bogus asylum applications and to remove failed applicants (p.80). Although they have achieved some measure of success in these fields, it has done nothing to staunch a flow of immigration that has now reached the level of a flood:

'…since 1997 asylum seekers have never comprised the majority of immigrants to Britain… there are four other principal ways by which lawful entry to Britain may be gained which have all increased markedly since 1997 as a result of government policies. These are: family reunion, including marriage; full-time study; through having obtained a work permit or some other form of authorisation to work here; and, finally, EU citizenship.' (p.81)

Immigration now adds one per cent to the population every two years

As a direct result of the policies of the present government, which amount to a virtual abandonment of the control of our borders, immigration is now running at levels which have never been seen before in our history. In 2004 and 2005 net foreign immigration was 342,000 and 292,000 respectively, representing an increase in the population of one per cent in two years. Compared with earlier waves of immigration like the Huguenots and the Jews, who increased the population by one per cent or less over a period of decades, it is clear that we are in an unprecedented situation.

Stability, freedom and tolerance under threat from immigration

David Conway argues that current levels of immigration raise questions not only about numbers but about integration - although the second is related to the first. Until the last part of the twentieth century Britain's immigrant population comprised only a very small proportion of the total population. As a result, in order to flourish they had to adapt to the prevailing culture and integrate. This has given Britain an enviable record of social harmony combined with considerable ethnic and cultural plurality. However, the presence of large ethnic communities, for some of whom integration with the host culture is not an aim, is threatening this social harmony.

Those who cherish Britain's comparative stability, freedom, and tolerance cannot afford to ignore the potential threat that is posed to it by the large-scale changes in its demographic composition now taking place as a result of recent large-scale immigration in combination with declining fertility among its indigenous population. A society must always find it harder to reproduce its political culture and to maintain its traditions the less deeply rooted its members become in it historically and ethnographically. Of late, there has been a growing realisation of the plausibility of some such claim in light of the discovery that all four suicide bombers of 7 July 2005 were British-born, second generation British Muslims who had grown up in Britain in highly segregated enclaves in which normal patterns of acculturation into mainstream British life have apparently become far harder to sustain. It is particularly in light of how quickly and recently many such enclaves have sprung up in Britain, and are continuing to grow apace, that all those who want to see Britain remain the stable, liberal, and tolerant country it has been for so long need to consider carefully how much truth or falsehood is contained in the claim hat Britain is and has always been a nation of immigrants. (p.6)

'A Nation of Immigrants? A brief demographic history of Britain' by David Conway is published by Civitas, 77 Great Peter Street, London SW1P 2EZ tel 020 7799 6677, www.civitas.org.uk, price £10.00 inc. pp.