Ethan Dennis, 21, who says he served in the Senior Service for four years, decided to travel east after Russia invaded.
The former seaman signed up on March 2, six days after Vladimir Putin's forces began their assault.
He says he went to the Ukrainian embassy but, unhappy with the red tape, decided to go it alone - flying to Rzeszów airport in southern Poland.
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From there he crossed the border into Ukraine with a group of ex-military Brits he met at the airport - spending 10 days fighting.
Ethan is now back in his Portsmouth home after spending time on the frontline.
And he has revealed just what he went through – including encounters with Ukrainian special forces and a Russian spy.
He said: ‘Having seen this brutal conflict firsthand, I hope that a deal between Russia and Ukraine is reached sooner rather than later - for everyone's benefit.’
Ethan said, upon arriving in Ukraine, the soldiers were bundled into the back of a van and driven to Lviv where they collected their gear and were assigned to a safe house.
After three days in the house, the group were surrounded by a Ukrainian special forces team, who'd been informed by a local of their position.
Ethan said: ‘At the safe house we were treated really well with comfy beds and good food.
‘But we believe a civilian informed the Ukrainian's about our foreign uniforms which raised suspicion amongst the higher ranks.
‘The team burst through the door and told us to put our hands on our head whilst they checked our documents and verified our stories.
‘One of our squad even had a gun put to his head when he refused to follow one of their instructions.
‘They had their safety [mechanism] off and their fingers on the triggers - it was an intense moment for us.
‘Once they realised we were there to help they left us alone.’
Unsatisfied with the lack of action they were seeing in Lviv, Ethan and his comrades decided to change squadrons and take the train to Kyiv.
They travelled to the capital, and, on their journey, helped police detain a suspicious man, who after being checked, was found to be making frequent calls to Russia.
The suspected spy was arrested and taken to a detention centre for further questioning – and Ethan carried on his journey.
When they arrived in Kiev, he says dozens of families sleeping in the tube station stood up to applaud them.
The squad were then driven to an undisclosable location where they were then briefed by an ex-major on conducting covert operations on the front line – just outside the city.
‘It was a surreal feeling getting off the train and having all these strangers start clapping you,’ Ethan said.
‘It felt nice to be apricated by these people who right now have so little to be enthusiastic about.
‘When we arrived at the briefing room with a different unit in Kiev we were handed better kit and proper military IDs.
‘It was a much more organised outfit than the unit we were with before, and within a day we could see why.
‘We were thrown right into the action and sent to guard a position just outside Kiev which was a key asset in the Ukrainian war effort.’
In the 10 days he spent in Ukraine, Ethan said he experienced missile strikes, mortar bombs and intense combat with Russian forces.
Thankfully, none of his British comrades were injured or killed during his spell on the front line – but several Ukrainian soldiers were not so lucky, he said.
Ethan, who says served in the military as an engineer, added: ‘Sometimes the Russians were so scared in contact they weren't even able to get rounds away.
‘There was an obvious lack of training amongst them and a lack of motivation to be there - which we used to our advantage.
‘The fighting was intense and very scary at points, but I just remembered all my training and tried to take calculated risks.
‘It was when the objectives we were being given became too high risk and too little reward that I decided I'd had enough.
‘Luckily I had signed an open contract with the major so he had no problem with me returning home.’
Ethan boarded a refugee train out of the capital and back to Lviv after his spell on the frontline.
When he arrived he was able to stay the night in a Red Cross tent, where he was woken up in the morning by the sound of shelling in the city.
After a gruelling walk to the border, he finally made it back over to Polish soil, but his journey was interrupted when Ukrainian secret police stopped him from crossing.
They were not convinced by his passport or his story of why he was in Ukraine, and they decided to detain him, he said.
Ethan claimed he finally managed to persuade the officer in charge to let him go, although throughout the rest of his time in Poland he was closely followed.
He then boarded a flight back to London on March 19.
Ethan, who was working as a carpenter before he flew out, said: ‘Getting home was really stressful.
‘There's always that doubt in your mind something may go wrong and you could be stuck.
‘The train ride to Lviv was the worst journey of my life.
‘At the Polish border I was stopped by Ukrainian secret police and detained because they didn't believe my story.
‘Once I'd convinced them, they still had agents follow me back to the refugee centre until I managed to get a taxi to the airport.
‘The relief I felt when the plane left that runway is indescribable.
‘My girlfriend was overjoyed to see me, and I've been told if I want to go and fight again she won't be there when I get back – so that's my decision made.’