The 'remain' or 'leave' EU referendum campaigns will kick into full gear from today with a tight battle expected.
If Mr Johnson does choose to campaign against the Prime Minister it would be seen as a blow for David Cameron who will already be up against leading Cabinet figures such as Iain Duncan Smith, Chris Grayling and Michael Gove.
The 'leave' advocates have been hoping Mr Johnson could be the figurehead their campaign has been looking for, with the ability to connect with voters in a way few other Westminster politicians can match.
Speculation that he is leaning towards the leave camp was heightened with the disclosure that he had a private dinner last week with Mr Gove, who declared on Saturday that he would be joining the "out" campaign.
However, Mr Cameron has been given an early boost with a poll by Survation for The Mail on Sunday putting support for remaining in the EU on 48% with 33% in favour of leaving and 19% undecided.
Mr Cameron will continue to make his case for voters to back his re-negotiation deal - finalised in marathon summit talks in Brussels - in a television interview with The Andrew Marr show.
In an interview with The Sunday Times (£), the PM warned EU "out" campaigners that Brexit will not stem the flow of migrants coming into the UK.
He said the EU would insist upon continued free movement of labour - as well as a contribution to Brussels' coffers - as the price of a free trade deal if the UK left.
"So far, the EU has never given full access to the single market without insisting on a contribution to the budget and free movement," he said.
The PM's words on migration immediately put him at odds with Chris Grayling - one of six ministers to announce he would be campaigning for "out" following Saturday's historic Cabinet meeting.
Despite the "emergency brake" on migrant benefits secured by Mr Cameron in his re-negotiation deal with Brussels, the Leader of the Commons said he saw no immediate end to the pressures on the UK.
Mr Grayling said only by leaving the EU could Britain put in place the measures it needed to control the numbers entering the country.