LONDON — In a week full of tests for the British prime minister, Wednesday's test is the most crucial.

Two days later than expected, Queen Elizabeth will set out the British government's legislative agenda on Wednesday at the official state opening of parliament.

For Prime Minister Theresa May, who lost her House of Commons majority after calling a snap election, the centerpiece of that occasion, the queen's speech, is a critical test of her authority. If she cannot get enough votes in the House of Commons to support the broad outline of her program then her government will almost certainly fall.

An event embellished with British regal pomp that usually passes as a formality has been infused with a sense of urgent political jeopardy. It could set the course for the Brexit government or bring May's premiership to a swift end, with another election the most likely result.

Here's POLITICO's guide to a key test for May's government — and what comes next if she cannot pass it.

What is the queen's speech?

The U.K. state opening of parliament — with its centerpiece in which the monarch reads out the government's legislative agenda — is a grand occasion steeped in tradition.

The delivery of a speech by the monarch can be traced back as far as the 16th century and the current ceremony dates from the opening of the rebuilt Palace of Westminster in 1852, following the 1834 fire. It marks the formal start of the parliamentary year and sets out the government’s legislative agenda for the coming session.

On Wednesday, Queen Elizabeth will be escorted in a scaled-back procession from Buckingham Palace to Westminster. This is not the first scaled back ceremony. The ceremony was also pared back in 1974 when Edward Heath called a snap election.

No horse-drawn carriage this time. The queen and the regalia of state and imperial state crown will arrive at the U.K. parliament by car. Queen Elizabeth will wear a day dress and hat rather than the usual robe of state and imperial state crown. The crown, sword of state and cap of maintenance — all symbols of the monarch's power — will be carried ahead of her by officers of state. The Duke of Edinburgh will also dress down, wearing a morning suit rather than his usual admiral of the fleet naval uniform.

Some traditions, however, will go ahead as usual. Each time parliament is opened, the symbolism of the House of Commons' independence from the monarchy is acted out as the House of Lords official, known as "Black Rod," is shut out of the Commons when he is sent to summon MPs. Black Rod strikes the door three times before it is opened and MPs then follow him and the House of Commons Speaker John Bercow to the Lords chamber to listen to the speech.

Queen Elizabeth will deliver a speech, written by the government, from the throne in the House of Lords — the upper legislative chamber.

What happens once the speech has been delivered?

When the queen leaves, a new parliamentary session starts. The British government announced Saturday that this parliament will sit uninterrupted for two years rather than the usual one, to give MPs more time to handle complicated Brexit legislation.

But its legislative program has to pass the House of Commons first.

Members of both the House of Commons and House of Lords will debate the contents of the speech. Six days have been set aside for the debate which will look at each different policy area.

MPs can table amendments to the speech and the debate concludes with votes on amendments. It is likely the crunch votes will take place on Thursday, June 29.

Will the Conservatives get their agenda through?

May needs all of her own Conservative MPs to back her program plus eight other MPs. At present Northern Ireland's Democratic Unionist Party (DUP) is most likely to provide the extra numbers. But as of Tuesday afternoon, there was still no deal between the two parties.

Conservatives insist they are "sufficiently confident" they will be able to get the queen's speech through the House of Commons. It would be hard for a unionist party to be responsible for toppling the government and paving the way for a Labour government under Jeremy Corbyn, who met with members of the IRA during the 1980s.

Corbyn, who is 64 seats short of a majority in parliament, said shortly after the election that his party would attempt to vote down the queen's speech. A source close to the Labour leadership said Tuesday that the Conservatives had no mandate for further cuts to public services and austerity. He said the party would use every opportunity to vote austerity measures down and table amendments to win support for policies in their manifesto.

"We stand ready to govern. The Tory minority government has no majority, no mandate, no authority, no governing program, so we are ready to take up the reins when the Tory administration falls apart," he said.

A rebellion on May's own benches is unlikely however. James Cleverly, a Conservative backbench MP, said there was a view in the party that being "gobby and rebellious" was not going to make things better.

"The opposition will table amendments that will try and divide us," he said. "They will try to put forward amendments to play silly games in terms of the relationship with the DUP and that kind of stuff. Most people both in parliament and externally will see those for what they are, which is game-playing rather than anything more substantive than that."

What if the queen's speech is voted down?

Traditionally the queen's speech is considered a vote of no confidence. The last time one was lost was in January 1924 when Stanley Baldwin called an election and lost his majority, but stayed on. When the subsequent king's speech was voted down he resigned and a minority Labour government took over.

However the 2011 Fixed-Term Parliament Act (FTPA), brought in by the Tory-Liberal Democrat coalition, changed things. It states that if a government loses a specifically worded vote of no confidence, the government would have 14 days to form a new government and to get a vote of confidence passed. Labour would then have a chance to form a government and try to win the confidence of the House of Commons.

While the FTPA was used for the first time to trigger May's snap election, its other clauses have not been tested.

According to Catherine Haddon, resident historian at the Institute for Government, it is not clear what would happen during the 14-day period following a vote of no confidence, and specifically if the government should resign to give Corbyn a chance to get a vote of confidence passed.

In theory the Conservatives could try and get another vote of confidence vote passed, and if they couldn't, they would force another general election.

Haddon said the "political reality" was that May would be under enormous political pressure to resign or would face an immediate vote of no confidence from Labour if the queen's speech is defeated.

"If Theresa May did lose the queen’s speech and felt compelled to resign immediately, then the question is whether Jeremy Corbyn would be the one to form a new government or refuse (refusals have occurred in U.K. government history), or whether the Conservatives would argue they could try again under a new prime minister," she said. "If Corbyn refused, either May would have to stay on or an alternative Conservative prime minister would have to form a government. Either way, two things are clear: the queen cannot be put in the position of making a choice between prime ministers and we cannot have no government."

And another election?

Haddon point out that if May offered her resignation immediately and Labour took office, it could not call an election (something the party has indicated it would like to do) without going through the FTPA motions that permit a government to call an early vote. Labour would still either need two-thirds of all MPs to vote in favour of another election — something that would require support from a large number of Conservative MPs — or would have to use the FTPA’s vote of confidence mechanism and vote down its own government.

"So, the FTPA could help the Conservatives stay in power. It allows them to say to the DUP that if they don’t help prop up the government for the queen’s speech, but also in any subsequent votes of confidence and the budget, a minority Labour government (or progressive alliance) could be formed," she said.

"However, political realities would play an important role. If neither the Conservatives nor a subsequent Labour government could manage to keep the confidence of the House of Commons, in the end there will be no alternative to a second election."