At the 35th British-French summit this week, UK foreign secretary Boris Johnson suggested building a bridge across the English Channel to connect the UK and France.

Johnson said, “good connections” were vital for the future.

At the moment, the two countries are connected via the Channel Tunnel, from Folkestone, Dover to Calais. It takes 35 minutes to travel 31 miles between the two countries.

However, the Channel bridge Johnson is proposing would be around 22 miles long.

How does the English Channel Bridge compare to other bridges?

The longest bridge in the world is the Danyang-Kunshan Grand Bridge in China. It spans 102.4 miles, roughly five times the size of the proposed Channel Bridge.

It is part of the high-speed railway in China which connects Beijing in the north to Shanghai in the south.

The longest continuous bridge over water is the Lake Pontchartrain Causeway in southern Louisiana, the US, which spans 23.83 miles.

How much would the English Channel Bridge cost to build?

If we go with the Lake Pontchartrain Causeway as a starting point, as this is a similar structure to what the Channel Bridge could be, building such a bridge would be very expensive.

The Causeway opened in 1956 and cost $46m to build. This equals around £33m. If we adjusted for inflation, using this very scientific calculator from the Daily Mail, that would be around £824m today.

English Channel Bridge

This isn’t too surprising. The estimated cost for a new bridge over Lake Pontchartrain would be in excess of $600m, making the £824m figure not unlikely.

This isn’t the first time a cross-channel bridge idea has been floated. According to BBC Archives, plans from 1981 showed that a submission was made for a three-lane motorway link across the English Channel. At the time, the plans suggested it would cost up to £3bn.

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According to the Daily Mail calculator, this would be around £12.3bn at today’s prices.

The Bridge

The Oresund bridge linking Sweden and Denmark (as featured in the eponymous crime drama) provides another clue as to the possible price of a future Boris Bridge. It cost £2bn and opened in 1999. The bridge spans five miles and then turns into a tunnel midway across the strait of Oresund. Doubling that figure and allowing for inflation would give a price of around £7bn.

Would it even be possible?

According to the UK Chamber of Shipping, the Channel Bridge wouldn’t be an easy feat to pull off. The representative of the UK shipping industry took to Twitter to warn that building the bridge in the middle of “the world’s busiest shipping lane” might come with challenges.

However, not everyone is pessimistic about it. Ian Firth, a bridge designer and former president of the Institution of Structural Engineers, told BBC Radio 4’s Today Programme that it would be a “huge undertaking, but absolutely possible.”

Downing Street has issued a statement to say there are “no specific plans” for a bridge linking the UK with France. The prime minister’s spokesman said:

“I haven’t seen any plans on that.”