Most wrongly assume that joint registration of the birth means they have legal status as the father and that, if the mother dies, they will automatically be granted custody of the child.

The report, published by the Joseph Rowntree Foundation, says few men realise they have to apply for a court-registered 'Parental Responsibility' agreement or a court order before they can be granted full legal rights.

Parental responsibility agreements must be sworn by both parties at either a country or magistrates' court.

Some fathers said they felt uncomfortable about asking for an agreement, feeling it implied a lack of trust in the mother.

If the mother does not give her consent, fathers have to seek a court order under the Children Act with the case being judged on merit.

Without full legal rights, fathers have no say in decisions such as whether their children should have certain types of medical treatment and even what they are called.

Sharp rise

The number of children born to cohabiting couples has risen sharply in recent years and they now account one in five of all births.

The majority of these are jointly registered.

But, out of 230,000 babies born a year to unmarried couples, legal agreements are only made in 3,000 cases with a further 5,500 being the subject of court orders.

The report's author Ros Pickford spoke to more than 200 fathers.

Most unmarried fathers only found out about their legal status if a problem occurred.

For example, one who took his child to hospital with heart problems discovered he could not sign the consent form.

Many reacted with anger, bewilderment or fear.

Most fathers saw marital status as irrelevant and the survey found there were few differences in lifestyle between married and unmarried parents.

The Lord Chancellor's Department announced last year that it was planning a change to the law which would give unmarried fathers who signed the birth certificate automatic parental responsibility.

But no date has yet been set for when the change will take place.

Ms Pickford says the government must act now. She also wants fathers-to-be to have better access to information about their legal situation.

Responsibilities not rights

She adds that many are confused because they are considered fathers when it comes to responsibilities such as paying child support, but not when it comes to legal rights.

Ms Pickford added: "Fathers in the survey felt their role was actually being undermined and devalued by this aspect of the law."

Fathers' groups have long campaigned for changes in the law on father's rights.

In response to a government paper on the situation, Familes Need Fathers state: "It is no longer tenable to maintain any distinction between the legal position of married and unmarried fathers in relation to their children."

It believes automatic parental responsibility for unmarried fathers should become the norm with the only exception being in cases where the mother has been raped.