Christian schools in England and Wales are under growing pressure to take pupils from different backgrounds because of a lack of places available in schools with no religious ethos.
The Association of Christian Teachers has told Premier that faith schools are constantly in a dilemma over whether or not to accept children of faith when a borough is overcrowded.
According to the spending watchdog one in five primary schools in England are now full or near capacity.
The National Audit Office has said the system is being put under strain by a population boom. It is claiming that more than a quarter of a million children, due to start school in England in the autumn of next year could miss out on a place.
A baby boom is being blamed for the rise in the number of children born in England between 2001 and 2011, the biggest 10-year increase since the 1950s.
Schools Minister David Laws has blamed the current crisis on the previous Labour government.
He said: "We've had a huge increase, tens of thousands of places more since the coalition goverment came to power and we are confident that inspite of the fact that the NAO are predicting that we'll need another quarter of a million places.
"We have got the money for that and we are going to deliver that."
Clive Ireson's the Director of Strategy at the Association of Christian Teachers.
He told Premier's Des Busteed on the News Hour the problem is approaching a crisis:
According to the National Audit Office, London has the greatest need, accounting for 37 percent of the extra primary places needed.
More than a fifth (20.4%) of primaries are full, or over capacity, while the numbers of children being taught in large infant classes of at least 31 pupils has more than doubled in five years. This has gone up from 23,200 in 2007 to 47,300 in 2012, the report says.
Siobhan Ryan is still waiting to hear if her son's got into a school of choice and described the current situation as unacceptable, saying: "They've obviously know this for a while, it's growing; it's not something that's happened over night.
"So they're should be things in place, schools should be getting bigger or expanding or building new schools, so they definitely should be doing something about it."
The NAO said it's uncertain how many places will be required in the future, but it is expected that more will be needed beyond next year.
Its report states that while the department considers that local councils are meeting their duty to provide enough school places, the watchdog found signs of pressure on the system.