New immigrants will add almost 1m people to Britain’s population over the next five years, creating huge pressure on public spending, a group of leading MPs has warned. Frank Field, a former Labour minister and the group’s leader, said the figures would give succour to the far right British National party (BNP) which was exploiting the failure of the mainstream parties to allay public fears about immigration. The BNP will receive a boost this week when it is given its first platform on BBC1’s Question Time.

The MPs’ warning comes as the Office for National Statistics (ONS) is expected to confirm that immigration will account for 900,000 of the anticipated 1.9m growth in the population over the lifetime of the next parliament.

Field, co-chairman of the cross-party group for balanced migration and a former welfare minister under Tony Blair, said that both Labour and the Tories had failed to address the population crunch. He believes the crisis in public finances caused by the recession means the government can no longer afford to pay for the extra schools, hospitals and other facilities needed. “It is obvious there will be no money to provide the extra facilities required by another 1m people,” he said. “The case for a really effective limit on immigration is overwhelming. Both parties must now decide on a limit and then tell the voters what it will be, how it will work and what it will cover.”

The ONS is expected to show that the population will grow by 10m to more than 70m in about 20 years. About 7m of the increase will be due to immigration. The MPs’ group argues that the only effective way to control immigration is to balance the numbers coming in with those going out.

Research published by Field’s group last month showed that by 2013 an extra 96,000 primary school places will be needed in England and Wales — the equivalent of nearly 500 primary schools. Two-thirds of those places would be for children with at least one parent not born in the UK. The extra burden on the taxpayer of building these schools would be £1 billion over five years, or £200m a year. Yet with a public debt of more than £800 billion, ministers are having to cut spending rather than increase it.

David Cameron has said that if elected a Tory government would introduce a cap on migrants, but he has yet to disclose a number or which types of worker would be affected.

Phil Woolas, the border and immigration minister, said: “Net migration has dramatically fallen. This is further proof that migrants come to the UK for short periods of time, work, contribute to the economy and then return home. “We have made it clear that we do not favour a cap on immigration because it is a crude measure which could harm the economy and is not as effective as our robust and flexible points-based system.”

Immigration is falling due to Eastern Europeans returning home because of the recession.

Field’s latest estimate assumes that net migration — the difference between those coming in to stay and those leaving permanently — will fall from 237,000 a year to 150,000 and stay there. Children born to immigrants will take the figure to 900,000 over five years. His group says net immigration would have to be cut to less than 50,000 if the population was not to increase to 70m.