One in 10 people in the UK attends church every week and one in seven goes once a month, according to research.
The survey found Northern Ireland was the most observant UK region
Christian charity Tearfund's survey of 7,000 people puts the UK among Europe's four least observant countries.
Two-thirds of those polled had not been to church in the last year, except for baptisms, weddings or funerals - but 53% identified themselves as Christian.
Tearfund said nearly three million more people would attend regularly if given the "right invitation".
It said churches could do more to offer encouragement to potential worshippers.
The poll, conducted last year among people aged 16 and over, suggests that one in four UK adults attends church at least once a year.
REGULAR UK CHURCHGOERS
Northern Ireland: 45%
Tearfund said 53% of people identified themselves as Christian, compared with almost three-quarters who had in the last census in 2001.
But it said that its survey indicated that three million people who had stopped going to church, or who had never been in their lives, would consider attending "given the right invitation".
This could be a personal invite, the chance to accompany a relative or friend, or the offer of help during difficult personal circumstances, it said.
Tearfund's president, Elaine Storkey, told BBC Radio Five Live that a lot of people would be unsure what to expect if they did visit.
"The church for a lot of people is a very strange place these days. They're not familiar with what's going on inside the building, with the form of service, with the way people gather, with what they say, how they pray.
"So the first thing they have really got to wake up to is that there is this big cultural gap between churched and non-churched."
The Reverend Lynda Barley, head of research and statistics for the Church of England Archbishops' Council, said Britain was a nation "seeking identity".
"At first glance, the past has been left behind to wholeheartedly embrace individual choice and secular consumerism prominently among its modern-day gods," she said.
"But research is beginning to show that there is more, far more, going on out of apparent sight in everyday life in Britain today."
The report, Churchgoing in the UK, found that Northern Ireland was the most observant region, with 45% of people attending services every month.
Regular churchgoing was also three times higher among adults from black ethnic groups than white.
National Secular Society executive director Keith Porteous Wood said the poll provided the "most authoritative evidence" yet that Britain had become an "overwhelmingly" secular society.
He said the survey showed that 40% of the population said they 'have no religion'
"It shows that two thirds of the UK population 'have no connection' with the church and that only a quarter 'believe in a personal god', just one of the several minimum requirements to qualify as a Christian," he said.
UK CHURCH ATTENDANCE
Regular churchgoers: At least once a month
Fringe churchgoers: At least six times a year
Occasional churchgoers: At least once a year
Open de-churched: Don't go, but likely to go again
Closed de-churched: Used to go, but unlikely to go again
Open non-churched: Weddings/baptism/funerals but may go again
Closed non-churched: Weddings/baptism/funerals but unlikely to go again
Other religions: Non-Christian
Unassigned: Did not answer question on prior church attendance