A former science teacher who is pedalling 13,000 miles across the width of the Eurasia said the challenge is becoming "more amazing by the day".

Jonathan Shubert, 26, set off from his Radlett home with university friend Imran Mughal, 33, on May 5.

The pair plan to cycle through eight time zones and 23 countries as they make their way - entirely unsupported - from France to Singapore.

Mr Shubert, of The Avenue, who is cycling between 100km and 200km a day, said: "It is amazing how much the people and the topography change in such small distances."

The two friends are currently tackling the Balkan Mountains, in Serbia, and they expect the whole journey to take six months in total.

Mr Mughal and Mr Shubert, who met as students at the University of Leeds in 2009, are regular cycling companions. It was after they cycled from Lands’ End to John O’Groats in 2010 that they became inspired to pursue other two-wheeled challenges.

Over the next three years they concocted a coast to coast journey from Calais to Beijing, which led to them naming the trip ‘cycling c2b’.

Mr Shubert said: "A fundamental idea behind the coast to coast journey was that the only mode of transport we would employ was the bicycle.

"After France, as we journeyed towards Italy, this threw up problems as we found snow blocking the mountain passes and our only alternative was to take the train."

This left the pair with one of their toughest challenges so far as, determined not to use any mode of transport other than a bicycle, they had to divert through Liechtenstein and Austria to reach the north of Italy.

Mr Shubert said: "This was demoralising and we did not speak for 30 miles as we descended back down the same mountain we had spent all day climbing up, to finish within 10 miles of our starting point."

He added: "From the ashes of our broken spirits, the phoenix of excitement and reinvigoration rose as an amazing group of unrelated Swiss families and a travelling circus came to our aid over the coming days to feed us, give us shelter and belief in the kindness of the human spirit."

The former Burnham Grammar School teacher and the former NHS employee - who both quit their jobs to pursue this challenge - are raising money for World Bicycle Relief and Charity Water - two charities that help impoverished people in some of the world’s poorest countries.

Mr Shubert said: "We like the fact we can see where every pound we raise is going and that the ideals of the charities will be viewed positively in all countries we travel through."

To track their progress, follow @cyclingc2b on Twitter.