An infertile man given the world’s first frozen testicular tissue implant has fresh hope of starting a family after he was able to produce sperm.

The now 27-year-old had tissue from his testicles frozen before undergoing toxic chemotherapy as a child to treat sickle cell disease.

He had the tissue re-transplanted last year - 16 years after it was removed - and tests show he is now producing mature sperm.

It is the first time a transplant of ‘cryopreserved prepubertal testicular tissue’ has been shown to restore sperm production in an adult patient.

Professor Ellen Goossens, who led the pioneering work at Vrije University Brussel in Belgium, said: ‘This is a huge finding. Many more people will have hope that they can have biological children.

‘It’s great to see for the patients for whom we already have tissue banked.’

Treatments such as chemotherapy and radiotherapy can be life-saving for childhood cancer and sickle cell patients but may also leave them infertile.

After puberty, it is possible to preserve male patients’ sperm to use later in IVF, but this is not an option for prepubescent boys.

In 2002, the Belgian clinic became the first to start banking testicular tissue of prepubertal patients.

The immature testes contain spermatogonial stem cells – the precursors of sperm – and sertoli cells, essential ‘nurse’ cells that support and nourish developing sperm.

‘At that time this field was in its infancy,’ Professor Goossens told the Guardian.

‘These methods were just being developed in animals. We told patients’ families we couldn’t guarantee that the fertility restoration would be successful.’

The clinic’s first wave of patients are now reaching their mid-20s and some have reached the point of wanting to start a family, including the first man to have tissue re-transplanted.

He had received high-dose chemotherapy in 2008 to wipe out his own blood cells before undergoing a bone marrow transplant.

Before the treatment, the clinic surgically removed one testicle, cut it into small pieces and froze the tissue.

Last year, four tissue fragments were grafted back into the remaining testicle and four under the skin of the scrotum.

After a year inside the body, the grafts were removed and analysed in a laboratory.

Two of the grafts from inside the testicle had produced mature sperm, which was collected and frozen.

‘The sperm that was isolated looked normal,’ Professor Goossens said, before cautioning: ‘We still have to see whether it’s able to fertilise an egg.’

Because the implanted tissue is not directly connected to the sperm duct, the researchers do not expect sperm cells to naturally find their way into the semen.

The patient is now considering whether to undergo a second round of grafts with the aim of collecting more sperm, or whether to proceed with IVF shortly.

The findings are yet to be peer-reviewed, meaning they have not been independently checked for accuracy and quality by experts in the field before being published.

Outlining their findings online, the researchers warn that pregnancy would require careful monitoring.

They wrote: ‘As the patient intends to pursue biological parenthood, close monitoring of embryo development, pregnancy progression and long-term health outcomes in any resulting offspring will be essential to ensure both safety and efficacy of this fertility restoration approach.’

Professor Rod Mitchell, a paediatric endocrinologist, is running a similar trial at the Centre for Reproductive Health at University of Edinburgh, which began banking testicular tissue in 2014 and, together with colleagues in Oxford and London, have frozen samples for more than 1,000 UK patients.

He told the Guardian he expects his clinic to carry out the first transplants imminently and added: ‘There is now proof of principle in humans that this approach is going to work, which is amazing.

‘I always believed it would work.

‘If you freeze tissue and keep cells alive, then they should have the potential.

‘You’re putting the tissue back into the perfect environment to stimulate it. Scientifically and biologically it makes sense. In reality, it’s still amazing.’

More than 3,000 patients worldwide already have testicular tissue banked. In the UK, it is estimated that about 200 patients each year would be likely to benefit.

Professor Mitchell added: ‘We’re at a point where, internationally, we’ve been working on this for 15 years – in some cases more.

‘It’s all been about collecting tissue from the boys. Now it’s coming to fruition. We’re all super excited about it.

‘One of the things we want to make sure is that people know about this. We know that we’re not necessarily reaching all of them.’