The last day of August marked the 22nd anniversary of the death of Diana, Princess of Wales. Reflecting on that time now can feel like looking at another country. It is difficult to imagine an event dominating the consciousness of the divided Britain of 2019 in the same way, or inspiring such a prolonged outbreak of state-sanctioned mawkishness. It is even harder to imagine popular opinion turning against Queen Elizabeth II, as it did for a few days in September 1997.

Headlines from the time are even more striking today. “Your People are Suffering. Speak to Us Ma’am,” said the Mirror. “Show Us You Care,” barked the Express. “Where is Our Queen? Where is Her Flag?” asked the Sun. And, though largely forgotten now, disquiet at the Queen’s apparent froideur came at a time of renewed uncertainty about the monarchy.

In January that year, ITV hosted a rancorous live debate on its future. One third of the 2.5 million viewers polled backed abolition. Public intellectuals proclaimed it dead. “The first British election ever without the monarchy,” Tom Nairn, the political theorist, wrote the week before Tony Blair’s landslide victory. “Is this not how it’s likely to be remembered?”

Could anything provoke similar dissent now? Until last week, it felt unlikely, if not unimaginable. The Queen, now into her 68th year on the throne, is 93. Her approval rating – 72 per cent, according to YouGov – is without parallel in almost any Western democracy. Her remarkable success in maintaining the power and prestige that so few European monarchies have is largely because of her ability to appear above the fray of electoral politics.

But after approving Boris Johnson’s request to prorogue parliament despite widespread Commons opposition to his Brexit policy – which came, like Diana’s death, in the last week of August, during her annual stay at Balmoral – the Queen is now at the centre of a constitutional storm.

She usually succeeds in avoiding them, despite occasional efforts to politicise her. Margaret Thatcher once complained she was someone who would vote SDP. David Cameron claimed she had “purred” after the pro-Union victory in the 2014 Scottish independence referendum. In 2016, Michael Gove was believed (by Nick Clegg) to have told the Sun that she had privately endorsed Brexit.

Yet the slights have never stuck, precisely because the constitution accords the Queen little agency of her own. She acts solely on the advice of her ministers and refrains from voicing personal opinions in public. A “golden triangle” of officials calibrates Whitehall’s handling of constitutional matters so as not to imperil her reputation for impartiality: Edward Young, her private secretary, Mark Sedwill, the Cabinet Secretary, and Peter Hill, Johnson’s principal private secretary. As such, there was never any prospect of the Queen heeding calls from opposition politicians to deny Johnson his prorogation. Nonetheless, it inspired a level of personal criticism unseen since that week in 1997.

Just as they did after Diana’s death, the public thronged to Buckingham Palace. This time, however, they came not to mourn but to picket. Public figures questioned her judgement. Jenny Eclair, the comedian, spoke for thousands of Remainers when she tweeted: “I’m really disappointed in the Queen.” Kate Osamor, the Labour MP, said she had failed to “save” the country from Johnson, and warned: “Look at what happened to her cousin Tino, ex King of Greece, when you enable a right-wing coup! Monarchy abolished!”

The backlash proved the success of her political project: appearing apolitical. John Sentamu, the Archbishop of York, articulated mainstream liberal opinion on the monarchy in 2015. It was a “bulwark against [the] kind of tyranny” he endured in his native Uganda under Idi Amin.

The problem for the Queen is that what we think we know of her politics is largely conjecture. With her public pronouncements limited to conciliatory niceties, it is easy for her subjects to convince themselves that she shares their view of what “tyranny” is.

As her ministers pursue a no-deal Brexit, the scope for controversy will only grow. Brexit pits the executive – and with it the Crown – against parliament, straining an already rickety constitutional settlement. Some in Downing Street believe that, should MPs legislate to prevent no deal on 31 October, the Prime Minister could advise the Queen to refuse royal assent. That would be unprecedented, but Johnson’s team has a taste for iconoclasm. Such a crisis, unlike the one after Diana’s death, could not be solved by media management.

It also throws the question of succession into harsh relief. How would Charles, with his more interventionist tendencies, respond to a similar challenge? Then there is the other royal crisis: the furore over Prince Andrew’s links to Jeffrey Epstein. The Queen has remained publicly supportive of her son, who denies any knowledge or involvement in Epstein’s crimes. That could still backfire. Though 2019 is not yet another annus horribilis, it might one day be remembered as the year that the most durable fiction of British politics – the infallible Queen – began to unravel.