A new route from London to South Wales was first envisaged in the 1930s, and proposals were drawn up for a series of local by-passes which could eventually be joined together to form a new route. The Berkshire County Council started to build a by-pass of Maidenhead, but work had to be abandoned in the early stages because of the outbreak of war in 1939.
After the war, the Special Roads Act 1949 gave highway authorities powers for the first time to provide roads restricted to certain classes of traffic, with access limited to a few specially constructed interchanges. The Minister of Transport decided that these powers should be used to provide the Maidenhead By-Pass, and the adjoining by-pass of Slough, as Trunk Road Motorways.
In 1956 the Government announced a plan for modernising the trunk road network based on five major projects, one of which was a road from London westwards to London Airport, the west of England and South Wales. The Maidenhead and Slough By-Passes were to be incorporated into this road, and construction of the Maidenhead By-Pass started again in 1959.
In 1960 the target of constructing 1,000 miles of motorway by the early 1970s was set. The London-South Wales Motorway was part of this 1,000 mile programme, and with the opening of the Maidenhead By-Pass in 1961 the first six miles of the M4 Motorway came into use.
Some six years later, in 1967, some 63 miles were open to traffic - 24 at the London end between Chiswick and Maidenhead Thicket, and about 39 at the western end between Tormarton in Gloucestershire and the Newport By-Pass in Wales. This western length included the Severn and Wye Bridges which were opened by Her Majesty the Queen in September 1966.
The 78-mile gap between Tormarton and the Maidenhead By-Pass remained to be filled. In June 1971 the gap began to close when 10½ miles between Tormarton and Stanton St Quintin were opened, and by November the motorway was extended another 18 miles eastwards to Badbury in Wiltshire, leaving only 50 miles to be completed. The opening of the 50 mile length between Badbury and Maidenhead By-Pass (Holyport) meant that there were 139 miles of continuous motorway between London and South Wales.