The European Tobacco Products Directive (TPD) stipulated that all e-liquids containing
nicotine sold in the EU must be notified to a designated authority. In the UK that authority is the Medicines & Healthcare Products Regulatory Agency (MHRA).

The Tobacco and Related Products Regulation (TRPR) was enacted to adopt the EU
regulations in the UK and so the regulations remain in force despite the UK having left the EU. 

Notified products are each assigned a European Community Identification (ECID) Number. Each version (nicotine strength) of an e-liquid is treated as a separate product.
The ECID number must be printed on the packaging of the product.

The notification, testing and identification of e-liquids was introduced to improve safety and to enable consumers to check that the products they are purchasing are genuine and have been tested. You will find a list of all notified products for Great Britain and Northern Ireland here.

E-liquids must undergo the following tests prior to being notified to the MHRA:

• Emissions testing: Products must undergo emissions testing to ensure they don't exceed safe levels of harmful substances, such as carbonyl compounds.
  
• Nicotine uptake testing: Manufacturers must determine and report the nicotine uptake per puff or per pack. Testing methods should be clearly documented, with a standard puff regime often recommended.
• Contaminant analysis: Testing must also include checks for potential contaminants and
  harmful degradation products, such as diacetyl, diethylene glycol, formaldehyde,
  acetaldehyde, and acrolein.
  
• Consistency: The consistency of the dose delivered per puff must be determined and
  reported.

E-liquid testing does not have to be conducted by an independent laboratory, but it must be performed in a way that is compliant with the regulations. Many companies use third-party testing labs because they have the specialized equipment and expertise required, but a manufacturer may also perform the testing themselves, if they have the necessary
capabilities. Testing must meet the standards set by the TRPR regulations.