The Grand Union Canal was a canal in England from Foxton, Leicestershire to Norton Junction on the Grand Junction Canal. It was bought by the latter in 1894, after which it became known as the Leicester Line of the Grand Junction. The larger Grand Junction Canal was bought by the Regent's Canal and from 1 January 1929 the whole network was known as the Grand Union Canal. For this reason, where clarity between the two Grand Unions is needed, the original Grand Union Canal is generally referred to as the Old Grand Union.
The Grand Junction Canal Company were concerned about these delays to the opening of the important route to the east Midlands, which would bring traffic onto their canal from the River Trent and the Nottinghamshire and Derbyshire coalfields. They requested James Barnes and Thomas Telford to revisit the question of route once again, and they developed a plan for a canal to link the part of the Leicestershire and Northamptonshire Union Canal which had been built with the Grand Junction Canal, which had been fully open since 1805.
Experience on the Grand Junction showed that broad boats caused delays as they could not pass in the tunnels, and so the Grand Junction was happy for the new canal to be built with only narrow locks, but with broad tunnels and bridges to allow passing of boats.
With the Leicestershire and Northamptonshire route unfinished, a Bill was put to Parliament to authorise a new canal, known as the Grand Union Canal, from the Leicestershire and Northamptonshire Union Canal at Foxton, to Norton on the Grand Junction Canal. The Act received the Royal Assent on 24 May 1810, entitled "An Act for making and maintaining a navigable Canal from the Union Canal, in the parish of Gumley, in the county of Leicester, to join the Grand Junction Canal near Long Buckby, in the county of Northampton; and for making a collateral Cut from the said intended Canal".
The terrain to be crossed was problematic for the canal engineers, as demonstrated by the several proposals made for routes. The core of the problem was the lack of river valleys or other obvious routes to take. The route taken by the Grand Union Canal involved a lot of twists and turns to maintain a level, though this does not keep to the contours as the main first generation canals of James Brindley did, but the difficult undulating countryside meant that the twists and turns were needed in spite of the cuttings and embankments. There were also two significant tunnels, at Crick 1528 yd (1397 m) and Husbands Bosworth 1166 yd (1066 m), both built wide enough for narrowboats to pass inside. From the junction at Foxton, the canal climbs dramatically through the ten Foxton Locks, and then is level for 22 miles (35 kilometres) before reaching the seven Watford Locks which lower the canal to the summit level of the Grand Junction Canal.
Benjamin Bevan was appointed as engineer. Despite problems with quicksand in the construction of Crick tunnel, the canal was completed in 1814, including a 1.6 mile (2.6 kilometre) branch to Welford, which also carried water from the reservoirs for the canal's water supply. The opening of the Grand Union thereby also provided an additional source of water for the northern summit level of the Grand Junction.
An inclined plane was opened at Foxton Locks in 1900, as part of a plan to enable wide barges to use the Grand Union Canal and thus bring more traffic onto the main line of the Grand Junction from the east Midlands. Widening of the locks at Watford was also planned, but not carried through.