Unlike Americans, Britons are not used to their Supreme Court being a focal point of political controversies. But the lengthy process to leave the European Union has changed all that, with Brexit raising not just legal questions that need answering — but also constitutional crises that need solving.

Britain’s highest court has made its boldest move this week, ruling unanimously that Prime Minister Boris Johnson acted unlawfully when he moved to suspend Parliament this month. The ruling means that British lawmakers could potentially derail Johnson’s plans for leaving the European Union and raises questions about whether he can continue as prime minister.

Critics say the court has been acting in a political manner, but many others have argued that it was in fact Johnson who overstepped his powers as leader of Britain’s government.

What does Britain’s constitution say about the Supreme Court’s powers?

Britain doesn’t have a written constitution as the United States and almost all other nations do. Instead, its constitution is scattered around different documents, including laws signed by Parliament, court judgments and, in some cases, simply convention, not written anywhere.

In practice, the key element of Britain’s constitution is parliamentary sovereignty — that lawmaking power is ultimately vested in members of Parliament, of which the prime minister is also a member.

This means that Britain’s Supreme Court is far more restricted in its powers than its American counterpart would be — it cannot strike down laws made by Parliament as the Supreme Court of the United States can, though it can ask Parliament to resolve laws it deems incompatible with already existing legislation.

When was the court founded?

Another key difference between the two courts — whereas the Supreme Court of the United States was established in 1789, Britain’s Supreme Court is a mere 10 years old.

Historically, Britain’s highest court had been the House of Lords, which in addition to being the higher chamber of the country’s legislature also contained judges to hear legal matters. However, the House of Lords’ legal powers were gradually shifted away, and a 2005 law removed the judges, known colloquially as “Law Lords,” from the court, creating a separate Supreme Court, instead.

The Supreme Court operates out of Middlesex Guildhall, which sits opposite the Houses of Parliament in Westminster.

How are the judges selected?

The court has 12 members, including the president and deputy president. The judges, known as justices, do not all sit on every case: They form odd-numbered panels of various sizes to prevent deadlocks.

When the court opened in 2012, the Law Lords were given places on the court. When spots open, judges are technically appointed by the queen on the recommendation of the prime minister, but the prime minister is required by law to offer a name given to him by a special selection commission made up of senior legal figures.

It is not a lifetime appointment: Judges are required to retire by age 70 or 75, depending on when they were appointed. In the past 10 years, 13 judges have retired, and one has died in office.

What role has it played during Brexit?

Britain’s June 2016 vote to leave the European Union has thrown up a number of high-profile legal problems for the court to consider. In January 2017, following a legal challenge by entrepreneur Gina Miller, the court voted 8-3 that Prime Minister Theresa May must get parliamentary approval for an agreement to withdraw from the European Union.

That case had a sizable impact. May subsequently tried to get Parliament to sign off on her deal four separate times — in each case, she failed. May resigned in July, which resulted in Johnson taking her place after a Conservative Party leadership contest.

This week’s decision may be more of a bombshell. Following another legal challenge by Miller, the court was asked to consider whether judges can intervene when a prime minister closes Parliament and, if so, whether it would support a lower court’s decision that Johnson had acted unlawfully in closing Parliament this month.

The decision was unanimous. Brenda Hale, president of the Supreme Court, said that Johnson’s decision to close Parliament (known as proroguing in British legal terms) was unlawful, as it “had the effect of frustrating or preventing the ability of Parliament to carry out its constitutional functions without reasonable justification.”

The court ruled that the move was “unlawful, void and of no effect” and that “Parliament has not been prorogued.”

What impact will this have?

In immediate terms, Britain’s Parliament will reopen on Wednesday, allowing lawmakers more time to push back on Johnson’s hard-line negotiating tactics for Brexit. Parliament will also fuel calls for a vote of no confidence in the British prime minister, possibly leading to a new election.

But more broadly, the result showed that Britain’s Supreme Court was willing to intervene in politics when it felt it needed to and to do so forcefully. With a unanimous ruling and an unusually frank judgment, the justices made clear that Britain’s Parliament was sovereign — in effect, helping to codify Britain’s unwritten constitution.