Johnson fronted the official "Vote Leave" campaign which won the referendum on Britain's EU membership, but Cummings is credited with playing a decisive role behind the scenes.

He was played by Benedict Cumberbatch in a TV drama earlier this year about the historic vote to leave the bloc called "Brexit: The Uncivil War".

Little known by the wider public, Cummings insisted on a data-driven social media campaign rather than traditional electioneering.

The tactics employed by Brexit campaigners have come under intense scrutiny since the referendum, particularly the use of misleading slogans and targeted political ads.

In July last year, the Electoral Commission watchdog fined Vote Leave for breaking campaign spending rules.

Investigators found that more than £675,000 ($853,000, 747,000 euros) spent with Canadian data firm Aggregate IQ via another campaign group should have been declared.

Since the referendum, Cummings has been a powerful critic of the government's Brexit strategy through regular opinion pieces in the centre-right magazine The Spectator.

In a series of tweets in 2017, Cummings said it was a "historic, unforgivable blunder" to begin the two-year EU withdrawal process without a detailed plan.

He said it was like "putting a gun in mouth and kaboom", he said.

Cummings has also been highly critical of some key Brexit supporters expected to be named to Johnson's team.

Earlier this year, he was ruled in contempt of parliament for failing to appear before MPs investigating fake news.

"The appointment of a man found in contempt of parliament is right in line with Boris Johnson's intention to suspend parliament to force through a damaging no deal," the pro-EU Best for Britain campaign group said in a statement.