[13] The existence of the operation leaked on 6 September 2018, when a press photographer captured a snapshot of a document revealing some "no-deal" plans and the HM Treasury codename for them.
[23][24][Note 1] Late on 21 March 2019, possible new exit dates were agreed between the UK and the EU: Consequently, full activation of Operation Yellowhammer was postponed until 8 April 2019.
[27][28] On 10 April 2019 the European Council granted the UK a six-month extension; Yellowhammer's 6,000-strong civil service team was disbanded a few weeks afterwards, with most members returning to their usual activities.
Developments since then – with Theresa May resigning as leader of the Conservative Party and both candidates to replace her talking of leaving without a deal by 31 October deadline – may make it necessary to resume preparations.
Tom Brake, the Liberal Democrat's Brexit spokesman said of the need for an extension, "if it is required, will be granted, leaving Yellowhammer nothing more than an expensive taxpayer-funded PR stunt.
[23] It will be organised via a Command and control structure (C3),[23] which will co-ordinate:[23] Major decisions will be taken by the European Union Exit and Trade (Preparedness) Sub-Committee, set up in January 2019[37] and chaired by the prime minister.
An example objective would be Continuity of supply of medicines into the UK after no deal exit from the EU falling within the areas of risk of key goods crossing borders and transport systems.
[23] 3,500 troops were placed on standby to 'assist the civil power' in the event of issues arising from a no-deal exit, although the Ministry of Defence had only disclosed their mission will be to "support government planning".
[5][38] On 21 March 2019, the British government's decision to risk a no-deal Brexit and to invoke Operation Yellowhammer was criticised by the First Minister of Scotland, Nicola Sturgeon.
The paper gave base-case planning assumptions in the event of a no-deal Brexit which could lead to food, medicine and petrol shortages, with a hard border in the island of Ireland, and a "three-month meltdown" at ports unable to cope with extra checks.
[46][47] In compliance, on 11 September the Government released a five-page document entitled "Operation Yellowhammer: HMG Reasonable Worst Case Planning Assumptions as of 2 August 2019".
[48][49] Other than a change of title – from "Base Scenario" to "Reasonable Worst Case Planning Assumptions" – and one redacted paragraph reportedly dealing with the impact on the oil refining industry,[50] the document was essentially identical to the one leaked in August.
The document was the subject of a two-hour long "Brexit readiness and Operation Yellowhammer" Ministerial statement and debate led by Michael Gove on 25 September, the day Parliament resumed session after the Supreme Court failed the prorogation attempted by Boris Johnson.
The petrol tariff was revealed by opposition members to be zero, and there was some concern expressed by Adrian Bailey and Melanie Onn about job losses at uncompetitive British plant.
[54][clarification needed] A report published by the Times of London on 10 August detailed the programme to help large employers, particularly in the construction and manufacturing sectors, because of their long supply chains.