Michael, who had been enjoying Freshers’ Week at Barts and The London School of Medicine, was soon diagnosed with Leber’s Optic Neuropathy, an extremely rare condition for which there is no known cause or cure.

Due to being twins, there was a 60 to 70 per cent chance of Daniel also losing his vision – a likelihood he describes as a “dark cloud” that hung over him during his first two years studying Aeronautical Engineering at the University of Bristol.

He had been preparing for his second year exams last Easter when he began to struggle to see through his left eye. Within two weeks his near perfect vision started to rapidly deteriorate.

Leber’s Optic Neuropathy is the loss of vision as a result of the death of cells in the optic nerve, causing it to stop relaying vital information from the eyes to the brain.

Although it’s a hereditary condition, linked to a number of genes, the brothers have been unable to find any family history going back four generations.

Only 150 people in the UK are known to be affected by the condition, with around 95 per cent never returning to full-time education.

Determined to think positively, the brothers have continued to study – Michael is a Geography undergraduate at King’s College London - and they now want to concentrate their efforts on raising money for Blind in Business, a charity which helps blind and partially sighted people into work through their training and employment services.

They are currently preparing for the 570km challenge, which begins on 6 April and will see them cycle on tandem bicycles from St Paul’s in the centre of London to Amsterdam in four days. They will be pedalling 140km a day, spending up to nine hours in the saddle, with the help of friends who will be leading from the front and their parents, Steve and Su-Jan, who are going to be driving a support vehicle.

Daniel, who has seven per cent vision now, said: “Leber’s Optic Neuropathy is known as ‘The Sword of Damocles’, based on the Greek parable, because your life goes from being great, in a period of heightened happiness, to hitting rock bottom in an instance. We soon realised it was extremely rare. An ophthalmologist will only see it two or three times in his career.

“Michael and I had been inseparable for 19 years until we both left for university. Then, in the first week of us being apart, he called me to say he thought something was wrong – he wasn’t able to recognise people and didn’t know why. That conversation still haunts me. Doctors initially thought he had a brain tumour but tests revealed that he had this rare genetic disorder.

“After his sight had almost fully deteriorated, specialists confirmed that he had this incredibly rare condition which meant the connection between his eyes and brain had affectively died. The whole family was devastated as we tried to comprehend what had happened. It was October 2009 – the cruellest of timing because it was my brother’s first term at medical school and he had his whole life ahead of him.

“The bombshell for me came when the consultant told me that I had a 60 to 70 per cent chance of also getting it due to the fact we are identical twins. I had a pretty tough time knowing that this dark cloud was hanging over me during my first two years at university. It was effectively like sitting on a time bomb that could have gone off at any moment. At least now I feel relieved because I know where I stand and can move on.”

Daniel’s sight unfortunately began to deteriorate just before his end of year exams but, without telling his tutors about his condition, he still managed to get a 2.1 and was determined to complete his degree and returned to the University of Bristol in October in a part-time capacity.

Daniel added: “There were some pretty dark moments when I had to come to terms with being severely visually-impaired. It was hard to come to terms with the fact that I could no longer do everyday activities. Going to the supermarket to buy food just isn’t possible now.

“Coming back to university has been tough but the last thing I wanted to do was sit at home doing nothing. My tutors at Bristol University have been really supportive and the university as a whole has been fantastic in how they’ve catered for such a change.

“Blind in Business has supported us in all aspects of my life, which is why we wanted to do something in return. Because it’s a small charity, we can decide where the money is spent so our aim is to raise over £3,000 and enable them to help others who find themselves in a similar position as us.”

Michael, who is now in the England Blind Football Squad, added: “Losing my sight at such an early age and watching my brother lose his as well has been an intensive process of rehabilitation, functionally and emotionally, which has put incredible pressure on the family.

“With sight loss, it’s a mental condition as well as a functional one as it involves a grieving process, mourning a loss that provokes great anger and shock. Needless to say, watching my twin brother – the person who’s closest to me - go through the same thing was extremely hard to overcome.”

Dan Mitchell, Training and Fundraising Manager at Blind in Business, said: “Having the Smith Brothers embark on such a challenging journey to raise money for this small charity shows they always want to work towards bigger challenges.  They have both been challenged academically and have pushed themselves as visually impaired people, working towards careers in engineering and law. 

“Blind in Business knows that these young men will challenge what visual impairment means and show how a group of people can work together in aid of a small charity to make a real difference to young visually impaired people.”

To sponsor Daniel, Michael and the rest of their team, please visit their JustGiving fundraising page.