Parliament will today seize control of the legislative agenda from the government for the first time in over a century.

It marks a hugely symbolic moment as the balance of power swings dramatically from the executive to MPs.

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The Week looks at what authority the British prime minister has, whether she has exceeded it, and if the country is headed for a dramatic realignment.

What powers does the prime minister have?

Although the government is officially titled Her Majesty’s Government of the United Kingdom of Great Britain andNorthern Ireland, power in the UK rests with the prime minister.

The PM must formally ask the monarch to form a government, but is “ultimately responsible for the policy and decisions of the government”, for appointing the Cabinet and setting the legislative agenda, explainsGOV.UK. This is normally based on the ruling party’s most recent election manifesto, effectively a blueprint for how it means to govern.

The prime minister is, in theory, accountable to the House of Commons but in most cases when the PM commands a majority, MPs’ primary function is to rubber-stamp legislation put forward by the government.

There are exceptions where the prime minister can act unilaterally without the consent of Parliament or even the Cabinet. One of these includes the power to declare war through what is called Royal Prerogative (using powers invested in the monarch but devolved to the prime minister).

As well as chairing all cabinet meetings, the PM also appoints or approves senior civil servants, ambassadors, the heads of the intelligence and security services, and some senior military staff.

Until recently the power to call an election was included, by asking the monarch to dissolve Parliament. However, since 2011 PMs have been bound by the Fixed-Term Parliaments Act which limits the ability to call a snap vote.

Was it always like this?

Parliament has functioned in one form or another since the Middle Ages, but it was not until the 18th century that the concept of a “first minister” began to take shape. The position was formalised by the MP Robert Walpole who shaped the office into something recognisable today. He was also the first resident of Number 10 Downing Street, which has remained the official residence and office of the prime minister.

At first the role was called First Lord of the Treasury and it was not until 1905 that the title “Prime Minister” was formally adopted and, since then, power has steadily concentrated in Downing Street. From the 1960s under Labour’s Harold Wilson, the prime minister’s office has steadily grown, with its own policy unit separate from other government departments. Over the past half century, the international role of the prime minister has also changed, vastly superseding that of the foreign secretary.

“The ‘presidentialisation thesis’ claims that the power of the prime minister is increasing so that he or she can no longer be seen as just the most important member of the Cabinet but is now more like an American president,” saysBrit Politics.

But unlike the US, Britain has no written constitution, meaning the exact role and authority of the prime minister is forever changing and growing.

While the US has a codified separation of power, “in the UK, those powers are consolidated”, writes Brittany Bennett inBustle, so it could be argued that Downing Street actually wields more power than the White House.

“Whereas the administrative branch is its own wing of the government, May is actually a member of the legislature, meaning that the two divisions operate as one,” says Bennett.

So why is this changing now?

Like its impact on the rest of the UK,Brexit has upended the accepted order of British politics.

Over the past two centuries the prime minister’s authority has in most cases been guaranteed as the leader of the largest party in the House of Commons. Even during hung parliaments, when no single party commands a majority of seats, minority or coalition governments have managed to function effectively.

But the breakdown of old party loyalties following the EU referendum, exacerbated by May’s disastrous decision to call the 2017 general election, which wiped out her working majority, has left the current PM in limbo – with more MPs questioning the “dictatorial” power of the executive during a time of national crisis.

During her time in office, May has effectively lost her parliamentary majority, suffered multiple Commons defeats – some of historic proportions – and seen a significant number of resignations from her government while in office.

“With the timetable now wrestled from her, May is in an incredibly weakened position and the numbers are such that she could even be considered the weakest prime minister in modern UK politics,” argues theIrish Independent.

What is more, according to the Attorney General Geoffrey Cox, anyindicative motion passed by MPs setting out a new route through the Brexit impasse would legally require the Government to follow Parliament’s instruction or risk breaching ministerial code.

“To put it another way, the PM would be obliged to endeavour to negotiate with the EU the revealed will of MPs, even if that revealed will involved a Brexit delay that requires the UK to participate in May's European parliamentary elections, or is at odds with the Tories' manifesto,” saysITV’s Robert Peston.

This would represent the ultimate transfer of power from executive to Parliament, setting a precedent that some claim could reshape British politics for years to come.