Dominic Cummings parted ways with Government over six months ago, but now he is back in the public eye and keen to dish the dirt on the Prime Minister. 

Boris Johnson took the Brexit campaign director with him when he entered number 10 in July 2019, and the following months saw the pair plot a landslide win in the General Election. 

Once the PM's closest advisor, Cummings was deemed the most powerful non-elected official. 

However, relations between the pair slowly deteriorated throughout the pandemic, culminating in Cummings leaving Downing Street in December 2020 clutching a singular box. 

At the time, onlookers speculated what secrets the contents of the box contained.

As it turns out, it was Cummings' phone, and in particular his WhatsApp, which held the most damaging evidence against the PM. 

In his first interview since leaving politics behind, Dominic Cummings is scathing of the Government's handling of the pandemic. 

But how did this once so powerful relationship turn so sour? 

Why did Dominic Cummings and Boris Johnson fall out? 

Cummings sudden exit from Downing Street came amid swirling rumours of upheaval and change to the Prime Minister's staff. 

Essentially, the advisor's departure was down to an internal power struggle. 

The Prime Minister's head of communications Lee Cain had worked with Cummings on the 2016 Brexit campaign, and was therefore a close ally of the advisor. 

In November, it was leaked that Cain was to be promoted to the PM's head of staff, a plan with which the PM's partner, Carrie Symonds, was reportedly displeased.

Soon after this story was leaked, Cain announced that he was standing down as director of communications and would be leaving number 10. 

This supposedly angered Dominic Cummings, who felt his ally had been forced out. 

From here, tensions continued to rise. 

Reports of briefings against Johnson's partner Carrie Symonds, conducted by Cummings, are said to have angered the PM, encouraging him to force his advisor out sooner.  

Indeed, just a day after Cain's departure, Cummings followed suit and was seen walking through the door of Number 10 carrying a cardboard box. 

Despite swirling rumours about his leaving, Cummings stated that it had always been the plan to leave by the end of 2020 and he would work the rest of his notice from home.

What has happened since his leaving? 

Since his departure, Cummings has posted many scathing blog posts about the Government's handling of the pandemic, as well as sharing WhatsApp messages between him and the PM. 

The messages included shocking revelations, such as the PM calling his then health secretary Matt Hancock "totally useless" and suggesting replacing him with Gove. 

One message also showed Johnson ask Cummings "Wtf do we do?".

He gave hours of evidence at a hearing laying out what happened in the run up to the pandemic in front of a committee of MPs, in which he was again highly critical of both Matt Hancock and the Prime Minister himself. 

He is expected to build on these allegations during tonight's interview, which will air on BBC Two at 7pm.