By Mark Easton
BBC News home editor
The NHS is spending £350m a year to provide maternity services for foreign-born mothers, £200m more than a decade ago, the BBC has found.
Immigration has raised the birth rate so fast that some units have closed, so that midwives could be moved to areas of urgent need.
A unit in Ascot, Berkshire, shut for two months in 2007 because staff had to be transferred to Slough.
The NHS says it is working to "build in" the extra capacity needed.
Other maternity units have turned expectant mothers away because they could not cope with unprecedented increases in the local birth rate.
When Labour came to power, the NHS spent around £1bn a year on maternity services, with one baby in eight delivered to a foreign-born mother.
Ten years on, spending has risen to £1.6bn, with almost one baby in four delivered to a mother born overseas.
While the number of babies born to British mothers has fallen by 44,000 since the mid-1990s, the figure for babies born to foreign mothers has risen by 64,000 - a 77% increase which has pushed the overall birth-rate to its highest level for 26 years.
In central London, where six out of every 10 babies born has a foreign-born mother, senior consultants and health managers blame the lack of resources to deal with the pressures of migration for unacceptably poor standards.
Professor Philip Steer, editor of the British Journal of Obstetrics and Gynaecology, said: "The Department of Health has been taken by surprise. The demographic change, the sheer numbers, has in some areas increased very substantially without there being any forward planning really to allow for that."
According to figures from the Office for National Statistics, in 2006 there were 15,000 more Eastern European babies born here than a decade earlier.
The statistics go on to show that 11,000 more babies were born to a mother from the Indian sub-continent, while 8,000 extra babies had mothers born in Africa.
Heatherwood Hospital in Ascot closed its maternity unit for two months in the summer of 2007 because of an "unprecedented increase" in the local birth rate.
Midwives were moved to Wexham Park Hospital, closer to the pressure-point of Slough where in the last year staff have witnessed an extra 150 babies delivered to foreign-born mothers.
In a statement at the time, local health officials said they "reluctantly took the decision to temporarily close the delivery suite at Heatherwood Hospital for two months so we can ensure we offer mothers-to-be a safe, high quality service at Wexham Park Hospital."
Tharlie's parents were told their local birthing unit was full
The knock-on effect was experienced in nearby Reading where the local maternity unit could not cope with the extra demand.
Tharlie Cooper was supposed to have been born in Reading, but when mother Lavina went into labour two weeks overdue she was told that, despite her being booked in, her local birthing unit was full.
Tharlie's father Dean was furious. "Basically we got turned away and the reply I got on the phone was wherever you ended up is where you end up", he said.
He drove his wife to Basingstoke in neighbouring Hampshire where doctors conducted an emergency caesarean.
LIVE BIRTHS BY COUNTRY OF BIRTH OF MOTHER
Births by British-born mothers down 44,000
Births by all foreign-born mothers up 64,000
Births by mothers born in Eastern Europe up 15,000
Births by mothers born in Indian subcontinent up 11,000
Births by mothers born in Africa up 8,000
Lavina, herself a migrant from South Africa, believes she and her baby could have died without prompt medical help.
The Royal Berkshire Hospital has apologised to the Cooper family.
In a statement the trust said: "It is a very difficult decision to close the unit and one which is not taken lightly.
"We are undertaking a strategic review to look at what sort of maternity services should be provided over the next 10 years, based on future population growth and birth rates."
How to cope
Peterborough has seen a huge increase in births from Eastern Europeans. There were just three such babies in 2000, but almost 200 in 2006.
At the Thistlemoor Medical Centre, births among patients have increased 33% in just two years. GP Nalini Modha fears the authorities have not planned for the new arrivals.
"Hopefully somebody who is in authority is actually looking at the figures to try and work out how they're going to cope with the influx" she said.
"If you're going to provide responsible care for all the population - the indigenous as well as the newcomers - then we will have to stop and think about what we can and can't afford."
In parts of Greater London, seven out of 10 babies are now delivered to mothers born overseas.
The Strategic Health Authority argues that this partly explains why maternity services in the capital performed so poorly in last week's Healthcare Commission report.
Births within migrant groups can often be more difficult, more dangerous and more expensive - with much higher rates of type 2 diabetes, tuberculosis and HIV among mothers who often turn up very late in their pregnancy.
London's chief nurse, Trish Morris-Thompson, admitted that the NHS had not realised how immigration would affect maternity services.
"The timing of the impact is much quicker than we had anticipated", she said.
"We're working with our commissioners and our maternity providers now to ensure that we're building in the capacity they need."