Campaigners and health professionals are concerned teenagers may be put off seeking sex advice by proposed changes to confidentiality laws.
Many youngsters said the proposals would put them off seeking help
The government is consulting on if it should require health workers to tell police or social services about under age sex to improve child protection.
Teenagers are currently entitled to the same confidentiality as adults except when they are at risk of serious harm.
A Brook survey found such a move would deter youngsters from seeking help.
The poll of 729 under-25s found that 64% would be less likely to seek advice on issues such as contraception, pregnancy and sexually transmitted infections if they knew professionals could pass on information. The figure rose to 74% for under 16s.
Brook chief executive Jan Barlow said confidentiality was the number one concern for young people when it came to sex advice.
"Any erosion of young people's rights to confidential sexual health advice and treatment would be disastrous.
"It could reverse all the good work that has been set in progress, leading to a whole generation of young people losing faith in the sexual health services available to them and a massive increase in the rates of unplanned pregnancies and sexually transmitted infections."
The charity said anecdotal evidence suggested some areas of London and Yorkshire had already started changing practices even though the Department for Education and Skills (DES) is still consulting on the proposed changes.
CASE STUDY: NAZIA, 15
"I think confidential advice should be available when you're in your teens because as much as we like to deny we need help, everyone needs someone to talk to. Especially someone we can trust to keep things confidential. But if you're pregnant and you go to a doctor without your parents I don't think they take you seriously. They think we're unable to make our own decisions. If a girl wants to have an abortion and she's underage I think as long as she's mature enough to make that decision, then she should be allowed to."
It comes as a mother is challenging the law by going to court in November to demand the right to know if her daughters have an abortion.
Brook has also launched a campaign, Wise Up!, to safeguard teenagers' confidentiality rights.
Their calls have been supported by a range of health professionals and campaigners.
Dr Michael Wilks, chairman of the British Medical Association's ethics committee, said: "Mandatory reporting of non-abusive relationships threatens the trust that underpins the relationship between doctors and patients.
"This will deter young people from seeking medical care to reduce risks of pregnancy and sexually transmitted disease."
Royal College of Nursing general secretary Beverly Malone said young people needed to have "trust" in the people providing care to them.
Anne Weyman, chief executive of the Family Planning Association, added: "Fears about the confidentiality of sexual health services are one of the main reasons young people fail to seek professional advice."
But Paul Tully, general secretary of the Society for the Protection of Unborn Children, said while it was not always necessary for professionals to inform other authorities if advice was given, a distinction should be drawn when it came to treatment such as abortion.
The DES said the changes were being considered as part of the government's response to the inquiry into the Soham murders.
"We are seeking views on what guidance would be helpful in sharing information about under-age sexual activity, to better safeguard children and young people from harm."