A British adventurer who has spent almost three decades walking around the world has been dealt a crushing blow just months before reaching home after being refused permission to complete the final obstacle of his epic journey.

Former paratrooper Karl Bushby, 57, has covered 36,000 miles since setting off from Chile in 1998 with one strict rule: he would not use any form of mechanised transport.

Now, after reaching Belgium and preparing to arrive in Calais in September, his hopes of walking through the Channel Tunnel have been dashed after Eurotunnel rejected his formal request to use its service tunnel.

Instead, the Hull-born explorer has revealed he is prepared to swim the English Channel rather than break the rules of the challenge that has defined the last 28 years of his life.

Bushby said: 'I've only just been told of the decision and it's early days. Clearly, my team will need to have conversations, but if I have to swim across the channel then I am prepared to do that.'

He later added: 'I may have to start paddling again.'

Eurotunnel said it had examined Bushby's request but concluded it could not safely allow the crossing.

A spokesperson said: 'We are aware of Karl Bushby's desire to walk through the Channel Tunnel as part of his journey. We have carefully considered his request and explored a range of possible options.

'In this instance, regretfully we are unable to accommodate Mr Bushby's wishes.

'The Channel Tunnel's Service Tunnel is a dedicated safety and maintenance route. Closing the Service Tunnel for a period of 15 hours, or over two days, would pose a safety risk to passenger services and impact essential maintenance work.

'Access is also tightly controlled and any activity within it requires detailed planning and specialist support.'

The spokesperson added that the safety of passengers, staff and infrastructure remained the company's highest priority and said it 'must ensure that all activity within the tunnel meets strict operational criteria'.

They added: 'We would like to congratulate Karl on his incredible journey to date and wish him well as he makes the rest of his way home.'

The refusal is the latest challenge in one of the world's longest and most extraordinary expeditions.

Bushby originally believed the journey from South America back to his childhood home in Hull would take around 12 years. 

Instead, wars, geopolitical tensions, border closures and visa disputes have stretched the trek to nearly three decades.

Along the way he has crossed the Americas on foot, trekked through the dangerous Darien Gap, spent 18 days imprisoned in Panama after crossing a closed border, narrowly avoided deportation from Russia and battled through some of the planet's most unforgiving terrain.

He has also repeatedly taken to the water when geography left him with no other option.

In 2006 he crossed the Bering Strait, swimming between gaps in the ice while wearing an immersion suit.

Last year he became the first person to complete a 186-mile swim across the Caspian Sea, avoiding entry into Iran or Russia by spending weeks in open water.

Bushby first voiced concerns over the Channel Tunnel more than a year ago, fearing it could become the one obstacle that prevented him from completing his self-imposed challenge.

At the time he said: 'It would be pretty miserable if it was a no. Even the Russians let me through [their country], despite world tensions.'

Before making his formal application, Bushby had admitted he remained hopeful permission could be secured to use the service tunnel running between the two rail tunnels.

He had previously joked about the alternatives, saying: 'The first alternative would be to take hostages. And lastly, to swim.'

Despite the latest setback, Bushby remains determined to complete the final leg of his extraordinary odyssey.

When he finally reaches Hull, he plans to finish the journey at his childhood home, where his mother will be waiting to welcome him after almost 28 years on the road.