Queen Elizabeth has rarely been drawn into political crises. (Reuters)

Queen Elizabeth II has been drawn into the Brexit battle as it comes to the crunch, opening the politically neutral sovereign to potentially challenging positions for her role as a constitutional monarch.

The 93-year-old head of state approved Prime Minister Boris Johnson's advice to cut down the number of days parliament will meet before Britain is due to leave the European Union on October 31.

Experts say Queen Elizabeth had no option but to approve the request.

Mike Gordon, professor of constitutional law at the University of Liverpool, said Queen Elizabeth may now face dangerous situations further down the line following Johnson's move.

The constitution is unwritten, instead relying on precedent and convention.

"This definitely puts the queen in a potentially tricky position because it's drawing her into the most contentious and divisive political debate in the UK," he said.

Britain is a constitutional monarchy, meaning the sovereign has the right to be consulted, to warn and to encourage, but can only act on the advice of her ministers.

"It's the oldest rule in the constitution," said Robert Craig, a constitutional expert at Durham University.

Simply put, the checks and balances of power mean that the monarch has authority in name while the prime minister has effective authority.

The next steps may become trickier still.

Johnson's opponents want to pass legislation to stop a no-deal Brexit.

If they succeed, "there's the possibility the government might advise the queen not to give the royal assent -- and at that point we'll be in difficult constitutional territory," said Gordon.

"The convention she gives the royal assent to anything parliament will pass clashes with the convention she acts on ministerial advice."

The last monarch to refuse royal assent -- signing a bill into law -- was queen Anne in 1708.

'Constitutional outrage'

On the throne since 1952, it is hard to imagine anyone better-versed in the sovereign's duties than Britain's longest-serving monarch.

Unlike in other countries where the head of state might play a more active role in politics, British politicians are expected to sort it out among themselves before involving the sovereign.

Queen Elizabeth has rarely been drawn into political crises.

The closest she came was the 1975 Australian constitutional crisis when her governor-general John Kerr sacked prime minister Gough Whitlam. She refused appeals to get involved.

The current session of parliament has been the longest in nearly 400 years.

Johnson's opponents see the closure and reset as politically-motivated and wrongfully using the prime minister's powers by craftily involving the monarch in the Brexit saga.

Even John Bercow, speaker of the House of Commons, branded it a "constitutional outrage".

The move has put the sovereign in a difficult position, with some seeing her as having taken sides over Brexit.

Anti-Brexit campaign group Best for Britain tweeted: "If the Queen is asked to help, she would do well to remember history doesn't look too kindly on royals who aid and abet the suspension of democracy."

It later clarified that its statement was not "intended to wish her harm".