Deep cuttings was not an option at Shrewley, as it was in two other areas along the canal. This was because the route passed under the village. The plans also specified that the tunnel would be suitable for boats but not barges, implying that it would have been suitable for 7-foot (2.1 m) narrow boats, common on the Birmingham system, but not for wider craft. However, construction of the Grand Junction Canal, which would provide a link to London, was started at a similar time, and the committee wisely took the decision in April 1794 to increase the width of the tunnel to 16 feet (4.9 m), allowing wide barges to pass through it. The width of the bridge holes was similarly increased to 21 feet (6.4 m) in May.
Northern entrance to the canal and accompanying horse tunnel
The tunnel is 433 yards (396 m) long, built of brick and is wide enough for two narrow boats to pass each other, although it has no towpath inside. It officially opened on 19 December 1799 when the rest of the Warwick and Birmingham Canal was completed, although no trading took place until the following March. It became part of the Grand Union Canal in 1929 following the amalgamation of the canal company with several others, including the Grand Junction.
There are actually two tunnels: at the northern portal, the towpath splits off and rises on a ramp and then enters a tunnel of its own, until it reaches the road which crosses the tunnel; having crossed this the path then descends on another ramp to reach the southern portal. The interior of the tunnel is unusually wet and this has led to the creation of flowstone formations.