The European Aviation Safety Agency (EASA) was formally established on 27 September 2002 and become operational one year later. The main objectives of the EU were strongly supported by the aeronautical industry. It was to create a body capable of ensuring a uniform, high level of aviation safety and independent of national interests. This included the free movement of aeronautical products and the automatic recognition of certificates. EASA was given the power to carry out legally binding certification tasks, overcoming the limitations of the Joint Aviation Authorities (JAA).
In the UK, organisation approvals have transitioned from national requirements BCARs and JARs to European rules as Regulation EU No 748/2012. Part-21 subpart G of this Regulation concerns the production of aircraft or engines or other products. This certification of production organisations is for the demonstration of their capability and means to meet their airworthiness obligations. It’s also a way to confer specified associated privileges.
It is worth noting that organisation approvals issued or recognised by the UK in accordance with the JAA system, and valid before 28 September 2003, where accepted as complying with the above EU Regulation. So, for production organisations operating in Europe, the transition from the JAA systems to the EASA system was a progressive development.
Today, European National Aviation Authorities (NAAs) need to have the resources necessary to undertake the approval of organisations. Once initiated, this means that the NAA and approved organisation have an on-going relationship for, at least, as long as any of their products are in-service.
The EASA Form 1 is the Authorised Release Certificate released by a Part-21 POA holder for stating that a product, a part, or a component was manufactured in accordance with approved design data.
Post March 31, 2019, Regulation EU No 748/2012 will not apply in the UK. However, for continuity it may be adopted as part of a Withdraw Agreement and taken-up in UK legislation. In the case of a “no-deal” scenario the situation is unclear. Any of the privileges that come from a European POA remain in doubt. I assume we will have a National POA with National privileges that may or may not be accepted/recognised by others. With an accepted/recognised Authorised Release Certificate products are deemed airworthy. Without one they are not.
 British Civil Airworthiness Requirements (BCAR).
 Joint Aviation Requirements (JARs).
 Production Organisation Approval (POA).