On Jan. 31, 2020, the United Kingdom formally ceases to be a member of the European Union, 1,317 days after citizens of the U.K. narrowly voted to do so in their Brexit referendum.

So what happens now? Does it mean the U.K. and EU are officially divorced?

As with many relationships, it’s complicated.

Prime Minister Boris Johnson and other “leave” proponents will be keen to celebrate finally getting “Brexit done” after years of frustrating negotiations. But as an expert in the EU and Brexit, I believe that “it is only half-time,” as Irish Prime Minister Leo Varadkar put it. The thorny negotiations are about to start.

On Feb. 1, Brexit’s impact on the United Kingdom will be limited. Yes, the country will no longer be part of the European Union and its members of the European Parliament will officially step down.

But, until Dec. 31, the U.K. will remain in the union’s single market and customs union. It will continue to contribute to the EU budget. And it will still have to follow the EU’s rules and regulations. This is all part of the so-called transition period – which could last as long as three years.

By joint agreement, the EU and the U.K. can extend this transition period once for either 12 or 24 months. The United Kingdom, however, ruled out this option in December. So, absent a change on this front, the two parties will have fewer than 11 months to negotiate the future terms of their relationship.

This is an extremely tall order for such a short period of time. London and Brussels need to reach an agreement on their terms of trade, as well as a host of other sensitive issues. These include law enforcement and security, data sharing, access to fishing waters, aviation standards and safety.

Comprehensive trade agreements are notoriously difficult to negotiate. For reference, the recent EU-Canada accord took seven years. As long as the end of 2020 remains the deadline, London and Brussels could only realistically strive for a minimal deal before the U.K. is on its own, leaving many issues to be resolved for later years.

Furthermore, the EU is a formidable negotiator. European Commission President Ursula von der Leyen made it clear that the United Kingdom faces clear choices. The more London diverges from EU rules and regulations, the more restricted its access to the EU’s single market. This matters a lot as the EU currently accounts for 45% of all U.K. exports and 53% of all its imports.

Moreover, the trade talks will have far-reaching ramifications. Once out of the EU, the U.K. will have to negotiate trade agreements with other countries, especially the United States – which is, after all, the United Kingdom’s second-largest trading partner after the EU. And the very different approaches to food regulations in the EU and the U.S. could hinder Britain’s ability to reach an agreement with both key partners.

Additionally, any other third country will first want clarity on the future terms of the EU-U.K. trade relationship before embarking on serious negotiations with Britain.

Finally, in a broader sense, leaving the European Union will present a stark challenge for the future of the United Kingdom’s foreign policy. London will reclaim some independence of action, but it could struggle to exercise influence in a new era of Great Power rivalry involving China, the EU and the U.S.

Thus, far from having completed Brexit, the United Kingdom will still face major decisions and challenges in the coming year.

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Garret Martin receives funding from the European Union for the Transatlantic Policy Center, a Jean Monnet Center of Excellence based at American University.