The new section was opened by Transport Minister Lord Adonis
A road upgrade allowing drivers to travel from Glasgow to London entirely by motorway has been officially opened.
The £174m scheme replaces a section of the A74 dual carriageway from Junction 44 of the M6 at Carlisle to the start of the A74(M) at Guards Mill.
The "Cumberland Gap" or "missing link" was a notorious bottleneck and the scene of many accidents.
Transport Minister Lord Adonis opened the new section on Friday, marking the 50th birthday of motorways in the UK.
During the work engineers faced a number of challenges including building a new bridge to take the motorway over the River Esk, and building a new viaduct over the West Coast railway line.
Care was also taken to protect animals and the landscape, with lizards and snakes relocated, and badger setts protected.
The scheme included a road being built alongside the new motorway for non-motorway traffic.
This means that farm vehicles, local buses, walkers or cyclists will no longer have to mix with high speed longer distance traffic.
The opening is half a century to the day after the first motorway-standard road, the Preston Bypass, was opened by then Prime Minister Harold Macmillan.
Lord Adonis said: "It is a fitting tribute to the pioneers whose foresight began the motorway era exactly 50 years ago that the very same motorway is today being extended between Carlisle and Guards Mill.
"This will ease a notorious bottleneck and improve safety."