Boris Johnson was swept to a landslide victory in the election on December 12, a time before anyone had heard of Covid-19.

On Saturday, 100 days from the election, the Prime Minister will be presiding over a country subjected to unprecedented peacetime restrictions and battling the biggest public health crisis in a generation.

This was not how it was meant to be for Mr Johnson, who secured his place in Number 10 on a promise to “get Brexit done” and revive a spirit of national optimism.

His key election promise was delivered on January 31, as the UK left the European Union.

With Big Ben’s bongs silenced by repair work, it was left to a light show on 10 Downing Street to mark the occasion – although the terms of the transition deal meant there was little noticeable change.

Talks have begun on striking a trade deal with the EU, although early exchanges have laid bare divisions on issues ranging from fisheries to a “level playing field” on workers’ rights, environmental standards and state subsidies to ensure fair competition.

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An image of Big Ben projected onto 10 Downing Street, London, as the UK left the EU (Aaron Chown/PA)

Despite the coronavirus crisis, the Prime Minister has insisted that he will not extend the transition period, meaning the UK risks leaving the transitional arrangements without any permanent trade deal at the end of the year.

After securing an 80-seat majority at the election, Mr Johnson had the political capital to reshape his Government in a reshuffle designed to reward allies and promote fresh talent.

But a showdown with Sajid Javid saw his chancellor quit less than a month before the Budget after refusing an ultimatum from Mr Johnson to sack his entire team of aides and replace them with a joint No 10/No 11 unit.

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Boris Johnson being greeted by staff as he arrives back at 10 Downing Street after the election success (Stefan Rousseau/PA)

In a quietly devastating Commons statement, Mr Javid said chancellors had to be able to “speak truth to power” and “the arrangement proposed would significantly inhibit that, and it would not have been in the national interest”.

He also took a swipe at Mr Johnson’s aide Dominic Cummings, whose influence within Government has been clear.

“I do not intend to dwell further on all the details and personalities – the Cummings and goings, if you will,” Mr Javid said.

Ahead of the reshuffle, Mr Cummings had given a dismissive assessment of the talent in the Cabinet, suggesting that cartoon superheroes PJ Masks “will do a greater job than all of them put together”.

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Dominic Cummings has played a key role behind the scenes in Downing Street (Simon Dawson/PA)

Mr Cummings’s drive to recruit “misfits and weirdos” into the ranks of government advisers also caused controversy, with Andrew Sabisky’s tenure as a contractor cut short after past comments about eugenics and racial IQ emerged.

Mr Sabisky once suggested enforcing the uptake of contraception to stop unplanned pregnancies “creating a permanent underclass”, and claimed the benefits of a purported cognitive enhancer, which can prove fatal, were “probably worth a dead kid once a year”.

The contractor, who quit in February, had also suggested that black Americans have a lower average IQ than white Americans.

Also in February, Mr Johnson was dubbed a “part-time Prime Minister” by Jeremy Corbyn after spending time holed up in the Chevening grace-and-favour mansion rather than visiting parts of the country hit by flooding.

One of the reasons why Mr Johnson may not have been keen to be out on the road was confirmed at the end of February when the Prime Minister’s partner Carrie Symonds announced she was pregnant and the couple were engaged.

News of the pregnancy emerged just hours after Sir Philip Rutnam quit his post as the senior civil servant in the Home Office following a bitter public feud between Priti Patel and her officials.

Mr Johnson publicly stood by his Home Secretary, who has denied allegations of bullying staff in three Whitehall departments where she has been a minister.

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Prime Minister Boris Johnson and fiancee Carrie Symonds (Yui Mok/PA)

But those scandals and Westminster rows have been dwarfed by the challenge now facing the Government.

The Prime Minister chaired his first Cobra meeting on coronavirus on March 2 and the speed with which events have progressed have given a sense of a Government – understandably – scrabbling to keep pace with the challenge posed by a new threat and rapidly evolving science.

Within weeks, he had warned that “many more families are going to lose loved ones before their time” and given advice for every Briton to work from home and avoid pubs and restaurants.

The draconian restrictions felt a long way from the kind of politician Mr Johnson once said he respected – the mayor in the movie Jaws who kept the beaches open despite the threat of the shark, with fatal consequences.

Chancellor Rishi Sunak announced £12 billion of coronavirus support in his first Budget and then, less than a week later, that looked like small change as he set out a £350 billion package of loan guarantees, grants and tax breaks.

Mr Johnson, a biographer of Winston Churchill, said he had declared war on the virus.

“We must act like any wartime government and do whatever it takes to support our economy,” he said.