The first details of the controversial Brexit checks Boris Johnson insisted would not apply to trade across the Irish Sea have emerged, with mandatory paperwork for businesses in Great Britain supplying goods to Northern Ireland from January. An HM Revenue & Customs (HMRC) document marked “sensitive”, seen by the Guardian, reveals that firms in Great Britain will be obliged to complete three rounds of customs, security and transit forms on all goods. The electronic documents will need to be completed whether there is a Brexit trade deal or not and will apply to all suppliers. An 11-page slide presentation from HMRC, outlining the new system, states: “To achieve customs control, we need to ensure that all goods are presented and declared to customs.” Under a new “Goods Vehicle Movement Service” (GVMS) system, hauliers or the owners of the freight will be obliged to pre-lodge three types of electronic paperwork before getting on a ferry from British ports such as Liverpool or Cairnryan in Scotland to Northern Ireland. The first paperwork applying to suppliers is an import declaration form setting out the customs code or codes for all the goods being transported to Northern Ireland. Second, the supplier will have to complete a safety and security declaration, paperwork that is currently waived on all goods being sold within the EU’s single market. Suppliers will then have to provide their truckers with a transit accompanying document (TAD) which must remain with the vehicle at all times so the EU can be guaranteed that the load that departs Great Britain is the same as the one arrives. While the goods going from GB to Northern Ireland remain in the UK market, they are being treated as exports and require this paperwork because of the unique arrangements made in the withdrawal agreement to avoid a border on the island of Ireland. For months the prime minister insisted that there would “emphatically” be no checks on trade across the Irish sea, causing a spat in January with the EU’s Brexit negotiator, Michel Barnier. HMRC is planning to streamline the system so that the three elements can be collated and pre-lodged with the authorities, who will then generate a “goods movement reference” number or GMR for the haulier to present to the ferry operator. This will demonstrate that the cargo is being processed by customs and give port authorities information on how to treat arrivals. Some trucks will be given the green light to their destination, others may have to be processed for tariffs if they are making an onward journey to the Republic of Ireland and those carrying food, drink and animal products will be subjected to health and diseases checks. Business leaders in Northern Ireland have been pleading with the government for this detail since last October, when the special arrangements for Northern Ireland were agreed between the UK and the EU. But because the prime minister subsequently insisted there would be no checks it became politically challenging to detail the new procedures until last month, when Michael Gove confirmed there would be checks after all. Sources say HMRC has told business representatives they plan to trial the new system in November, which they feel is perilously close to January’s full implementation deadline. Labour’s shadow Northern Ireland secretary Louise Haigh branded the government delay in detailing the checks as “wreckless”. “Ministers wasted eight months insisting these checks would never be needed. With six months to go, businesses are still in the dark and the new system to manage these checks doesn’t yet exist,” she said. “This level of incompetence and failure to prepare would be irresponsible at any time but, right now, it is completely reckless. Jobs have already been lost and their cavalier approach risks costing many more.” The procedures will not apply on goods going from Northern Ireland to GB. HMRC said it would “ensure” the new processes would be “light touch via electronic submission” and would “impose the minimum possible burden” on traders. A government spokesman said: “Our approach, welcomed by businesses, ensures that Northern Ireland will benefit from unfettered access to the whole UK market and that there will be no tariffs for internal UK trade in any circumstances”.