By Hannah Richardson
BBC News education reporter
Sex education is to become compulsory from 2011
Ministers are being accused of caving into pressure from faith schools over sex and relationships education.
The government is due to make sex and relationship lessons compulsory in England's schools from 2011 through a bill in Parliament next week.
But pressure groups and the Liberal Democrats say a late amendment means faith schools will be able to teach it in line with their religious character.
The government denied faith schools were being given an opt out.
In a statement on its website, the Catholic Education Service says the amendment was secured after a period of "extensive lobbying".
It said the change would allow schools to teach the subject in accordance with their ethos.
The body would not comment further on the amendment.
Sex and relationship education - who does what?
Age 5-7 - puberty, relationships and how to keep safe
Age 7 - 11 - puberty, relationships including marriage, divorce, separation, same sex and civil partnerships and managing emotions and dealing with negative pressures
Age 11 - 14 - Sexual activity, human reproduction, contraception, pregnancy, STDs including HIV/Aids and high risk behaviours, relationships, including those between old, young, girls, boys and same sex
Age 14 - 16 - Body image and health, choices relating to sexual activity and substance misuse, and the emotional well-being, reducing risk and minimising harm, parenting skills and family life, separation, divorce and bereavements, prejudice and bullying
Instead, it pointed to a document on its website which said teachers would need to act wisely, "mindful that the teachings of the Church must be upheld in our Catholic schools and the innocence of children preserved".
It continued: "This must take place whilst also acknowledging pupils will often be encountering conflicting messages from external sources.
"This is an example of where good teaching will need to respond to the maturity of children and the environment in which they live, coupled with respect for the dignity of all human persons, upholding the Church's teachings."
Under the plans, all schools are to be required to teach children aged seven to 11 about relationships including marriage, same sex and civil partnerships, divorce and separation under Personal, Social, Health and Economic Education.
Secondary school pupils are to learn about sexual activity, reproduction, contraception as well as same sex relationships.
Clauses in the Children, Schools and Families Bill require schools to cover these issues in an "accurate and balanced" way, reflecting a reasonable range of religious, cultural and other perspectives and the religious and cultural backgrounds of the school's pupils.
The subject is also to be taught in a way that promotes equality, accepts diversity and emphasises both rights and responsibilities, the Bill says.
This requirement could have been problematic for schools governed by religions that are specifically opposed to homosexuality and contraception.
But an amendment to the Bill, tabled by the Education Secretary Ed Balls, suggests these principles could be diluted by head teachers and governing bodies of faith schools, so that teaching reflects a "school's religious character".
A spokesman for the Department for Children, Schools and Families denied faith schools would be able to opt out of statutory sex and relationship lessons when they come into effect in September 2011.
"All maintained schools and academies will be required to teach the full programmes of study in line with the principles outlined in the Bill including promoting equality and encouraging acceptance of diversity," he said.
"Schools with a religious character will be free, as they are now, to express the views of their faith and reflect the ethos of their school, but what they cannot do is suggest that their views are the only ones.
"The bottom line is that all young people must by law receive accurate and balanced information so that they can make informed, positive choices."
But Liberal Democrat Children's spokesman David Laws said the amendment was "a serious and undesirable U-turn" by Mr Balls.
"The government has already given an opt-out from sex and relationship education up to age 15.
"This looks like it will further water down the information which all young people should be entitled to before they reach the age of consent."
The Accord Coalition - which calls for an end to what it sees as religious discrimination in school staffing and admissions - also criticised the amendment.
Its spokesman Rabbi Dr Jonathan Romain said: "It is astonishing that the government plans to deny young people of their right to accurate, balanced PSHE and Sex and Relationships Education (SRE), and allow state-funded schools to teach the subject from one religious viewpoint."
Chief executive of the British Humanist Association Andrew Copson said the amendment effectively gave a licence to faith schools to teach SRE in ways that were homophobic, gender discriminatory and violated principles of human rights.
They both called on Mr Balls to withdraw the amendment.