"The campaign will not be along party lines: there will be a pro-EU grouping and there will be an anti-EU grouping," Sir Tony Brenton said.
Recent media reports cited unnamed officials close to Prime Minister David Cameron floating June 2016 as a possible date of the anticipated Brexit referendum.
Cameron himself made a campaign pledge to hold the "in-out" vote before 2017, and has advocated reforms within the 28-nation bloc ahead of the plebiscite.
Sir Brenton elaborated that both the ruling Conservative and opposition Labour parties contain opposing factions on the issue of EU membership.
Asked to predict the outcome of the Brexit vote, Sir Brenton said "my guess for the moment is that we are going to vote for staying in."
The diplomat explained that he expected the pro-EU vote to prevail because of Cameron's vow to negotiate reforms with Brussels.
These include requesting greater control in setting migration policy, slashing employment benefits to EU migrants and a more relaxed enforcement of EU legislation.
Cameron's pledge to hold the vote before 2017 is driven by rising Euroscepticism and anti-EU sentiment in the UK fueled by speculation that the UK's economy is being stifled by an influx of migrants from less affluent EU member states.
