With all eyes on the royal baby, there is no chance George Alexander Louis Windsor will ever be rendered invisible. The same cannot be said for the 51 million babies – almost one in three of all babies born across the world annually – whose births are not registered each year. These children do not have a birth certificate and, legally speaking, do not exist.

Globally, there are an estimated 220 million children under five across the world whose birth is not recorded. That excludes China, where figures are unknown. There is growing evidence that, without a birth certificate, such youngsters are more likely to be poorer than even the most disadvantaged of their peers, struggling to access healthcare, attend school, sit exams, or even get the vaccinations they need to survive.

With pressure mounting to meet the UN Millennium Development Goals – a set of targets to reduce global poverty and improve living standards by 2015 – experts say there is now "growing momentum" to ensure these hidden children's rights are restored. Seventeen countries in Latin America and the Caribbean intend to declare 2014 as the year of "Universal Registration".

"The position now is, if you are invisible, you are vulnerable," said Nicoleta Panta, advocacy manager at Plan International's universal birth registration campaign. "How can you measure progress on goals when not every child is counted? How can children be vaccinated if nobody knows they even exist?"

A child without a birth certificate, and therefore unable to prove his or her age, is more at risk of being exploited by being put to work, of being arrested and treated as an adult in the justice system, of being forcibly conscripted into the armed forces or child marriage, or of being trafficked, experts warn.

It is also almost impossible to open a bank account, get a passport, vote, or even gain employment, without a record of your birth. In certain countries, such as Senegal, birth certificates are required for enrolling a child in school and for registering to write exams. In Tanzania, it is required for university enrolment; in a number of Latin American countries, you need to legally exist before you can take secondary-school exams.

But despite the need, the problem shows no sign of dissipating. National plans for registration were drawn up for 24 countries in central and western Africa in 2004, but more than 60 per cent of children in sub-Saharan Africa still go unrecorded.

There are six countries in the region where less than 10 per cent of babies are registered, according to Unicef. This includes Somalia –where just 3 per cent of children are registered at birth. South Asia is another "hotspot": only 37 per cent of babies are registered. In Bangladesh, this figure drops to just 10 per cent.

Afroza, 18, a mother of 11-month-old twins, Diya and Habib, knows at first hand what affect this can have on your life. Her birth was not registered and, with no official record, her parents were able to obtain a fake birth certificate, falsifying her age. While the legal minimum age for marriage in Bangladesh is 18, she was wed at just 15.

"At 16 years old, I was carrying two babies in my body," she said. "My body is so small; I don't know how I did it." She struggled to eat after giving birth and has developed a fear of vomiting since the twins were born. Breastfeeding her babies was also difficult due to a lack of breast milk. "I feel inadequate as a mother. I cannot provide [enough] for my babies," she added.

Kendra Gregson, Unicef's senior child protection officer, said a lack of identity would be a "persistent problem" affecting unregistered people throughout their life. "It's the first document that says you exist before the law. Not having a certificate puts you on one of those steps towards being stateless. Birth registration should be universal and should be free," she said.

Most developed countries have universal registration, except for the Balkans, where almost none of the countries has a 100 per cent registration rate. In England and Wales, 96.5 per cent of parents comply with the 42-day registration rule; the 3.5 per cent – or more than 25,000 people – who do not could face a £200 fine.

But, according to Plan, almost 100 countries worldwide do not currently have "functioning systems" to meet their registration needs. "There are a couple of key barriers," Ms Panta said. "Places don't have the right technology or capacity, or it can be difficult for mothers to understand the benefit of a birth certificate. High costs can be incurred through travel or accommodation and loss of earnings when registering."

Certain countries have tried to tackle these barriers head on. Places such as Nigeria, Senegal, Uganda and Kenya have all launched initiatives to register babies via SMS; while New Delhi, in India, has achieved universal cover by combining the online registration of births and deaths and linking them to immunisation. Other places, such as Bangladesh and Tanzania, have implemented ID cards.

Brazil, meanwhile, is tipped as one of the world's success stories in this field.

It made registration free more than a decade ago, and then placed units within maternity wards in states with the lowest rates of birth registration. Rates went from almost 70 per cent in 1995 to more than 90 per cent today.

Baroness Tonge, chair of the All-Party Parliamentary Group on Population, Development and Reproductive Health, said birth registration was a priority.

"No one knows these people exist officially," she said. "We have to protect future generations and make sure these registrations take place."

Case study: 'Police always stopped me from leaving my village since I didn't have anything to prove I was Thai'

Joe, 18, from Chiang Rai, Thailand, only received his Thai citizenship and identification card last year, after a DNA test. His parents, who belong to the ethnic Akha hill tribe, were still waiting to be confirmed as citizens when he was born. Joe, like more than 50,000 people in his province, did not exist on government records and was effectively rendered "stateless" in the country of his birth. His parents had to pay for his education, while his peers got it free.