From a EU customs union history:
EU adopts the Community Customs Code. It assembles for the very first time in a code the provisions of customs legislation that were previously contained in a large number of Community regulations and directives - a job of fundamental importance for the internal market and a paramount simplification for traders and customs.
Free movement of goods is now a reality. No more customs formalities at internal borders of the EU. No more long queues for commercial vehicles for goods to be declared and/or checked at border crossings. For the first time, uniform customs legislation becomes directly applicable in all Member States of the EU.
From another EU presentation
The customs union came into being 18 months ahead of schedule on 1 July 1968, when the remaining customs duties on intra-Community trade were abolished and the common customs tariff was introduced to replace national customs duties in trade with the rest of the world.
In 1993, customs controls at internal borders were abolished and long queues of commercial vehicles at border crossings are now a distant memory.
So given the noteworthiness of this removal of internal checks, in general terms: which code or treaty provision enabled the 1993 elimination of the internal customs checks?
It's easy enough to trace that broadly to the SEA:
In 1993, the Single European Act came into full force creating a single European market without frontiers. The European Communities were renamed the European Union. This meant the removal of all duties and restrictions as well as procedural barriers and checks by customs administration at internal frontiers.
Although the SEA was ratified in 1987, the 1993 date was seemingly set in it. In a little more detail
By amending the original 1957 Treaty, the EEC gained the enabling instrument for the Single Market. The revised Treaty – the Single European Act - came into force in July 1987. Its provisions included: [...]
- gradually establishing the Single Market over a period up to the end of 1992, by means of a vast legislative programme involving the adoption of hundreds of directives and regulations;
With these changes in place, 282 laws were passed between 1985 and 1992 to sweep away the technical, regulatory, legal and bureaucratic barriers that stifled free trade and free movement. The Single Market was finally put in place on 1 January 1993.
But which of those laws (directives, regulations or codes etc.) specifically did away with the internal customs?
Interestingly, the Council passed regulation 3904/92 which set rules what to do with the former internal customs agents... like what financial assistance to offer to those laid off since many of the checks are no longer needed. But that doesn't cover the core issue of closing down the internal customs posts.
As a good search hint from there is that many such time-synchronized regulations contain the text "It shall apply from 1 January 1993". But that already produces a lot hits...