LONDON (Reuters) - Britain’s status quo transition period with the European Union will end at 2300 GMT on Dec. 31. Prime Minister Boris Johnson said on Thursday there was “a strong possibility” Britain and the EU would fail to safeguard free trade past the end of the year. If the two sides cannot agree a deal, there is a risk that flights between them could be grounded, but the EU announced contingency measures on Thursday aimed at preventing this. CURRENT DEAL Under current arrangements, UK and EU airlines can fly any routes they wish within the bloc but after the end of the transition period, there will be no legal framework for flights to continue to operate between the two sides. BASIC CONNECTIVITY Whether or not there is a deal, experts say neither side wants to see flights grounded or disruption to travel. The EU laid out “basic air connectivity” contingency plans on Thursday to ensure the provision of “certain air services” for six months, provided that the UK ensures the same. Confirmation from the British side is expected shortly, given that British transport minister Grant Shapps said in mid-October that the UK would look to reciprocate any basic connectivity measures put forward by the EU. “The UK and EU have a mutual interest in maintaining vital connectivity,” a British transport department spokeswoman said. “We are reviewing the detail of the EU’s regulations.” ACCESS CUT Under a basic connectivity deal, airlines would likely have their access to their non-home market restricted, by comparison with their current freedoms. That deal would allow carriers to fly from the UK and Europe and vice-versa, but it might prevent a UK carrier from flying between two European cities, or a European carrier flying between two UK cities, for example. AVIATION WITHIN A TRADE DEAL Should there be a trade deal, Johnson said on Oct. 16 that agreement on aviation issues was advanced. “A lot of progress has already been made, by the way, on such issues as social security, and aviation, nuclear cooperation and so on,” he said at the time. The details of that trade deal will need to be revealed before the implications can be understood for airlines. OWNERSHIP RULES UK airlines have spent the last four years making plans to prepare for the new relationship between the pair. EU rules state that airlines must be at least 50% owned by EU nationals or risk losing their operating licences. Britain-based EasyJet set up a new airline in Austria in 2017 to protect its EU flying rights. Currently, the company is 45% owned by non-UK EU nationals and it is trying to add more of those shareholders to reach the 50% threshold. IAG, which owns British Airways, Iberia and Aer Lingus, is a Spanish-registered company headquartered in Britain. It says it is confident that it will comply with EU and the UK ownership and control rules once the transition period ends. COVID-19 Prior to the COVID-19 pandemic, 164 million passengers travelled between the UK and the EU each year, but that figure has shrunk by more than 80% as lockdowns and travel restrictions have grounded huge swathes of the industry. Traffic is expected to pick up over the Christmas period, in what could be the start of a much-needed travel rebound for cash-strapped airlines. A basic connectivity deal is the minimum required to underpin any recovery. U.S. TRAVEL Britain signed a deal with the United States last month to allow the continuation of flights, agreeing a new framework to replace the EU-US Open Skies Agreement which will continue to cover the UK until the end of the transition period.