The Constitution Unit at University College London said a fresh poll would require at least 24 weeks – taking the UK right up until the March 2019 exit date.
Their verdict is a blow for anti-Brexit campaigners who have been calling for a referendum on the final deal Theresa May strikes with Brussels.
Mrs May has insisted there will be no second referendum under any circumstances, while Labour has left the door open to a fresh vote if Parliament ends up at an impasse.
According to the UCL team, a second poll would be possible but not without extending the two-year Article 50 process, which would require unanimous agreement from the other 27 member states.
The team said around 12 weeks of parliamentary time would be needed to prepare the relevant legislation and test the proposed question before a 10-week official campaign period.
It said if Parliament began drawing up the legislation today, the process – including a two-week Christmas recess – would take the country up to 28 March next year.
And it added: “There is, of course, no chance that legislation would be introduced on this date. It is also unlikely that all the conditions necessary to hold a referendum according to the minimum timescale would be met.
“And a referendum on the day before exit day is anyway not practicable. Therefore, an extension to the Article 50 period to delay exit day seems necessary to allow a referendum to be held.”
In its own analysis of the timetable for a second referendum, the People’s Vote campaign said its priority would be to force a new poll “before the Article 50 deadline expires”.
But it admitted: “Although it is possible to hold a vote before March 29th, the Government could consider obtaining an extension of the Article 50 timetable to allow time for the process to play out.”
Lord Kerr from the People’s Vote campaign said the UCL report proved there were “no insurmountable hurdles” to another referendum “if the political will is there”.
The peer – who authored the Article 50 treaty which governs how EU states must leave the bloc – added: “Informed public consent is essential.
“The negotiation process means that we will all be much better informed when it ends than we were in 2016. The outcome should be put to the people.”
Elsewhere, the UCL report said MPs could trigger another referendum by voting down the final Brexit deal in parliament or making their backing of a deal conditional on another public poll.
It also said any new referendum would be best to take a three-question format, with the public offered to remain in the bloc, to quit with the deal struck by the Government or to quit with no deal.
Meg Russell, the director of the UCL Constitution Unit, said: “In the British system anything is possible given the political will.”
She added: “As events remain unpredictable, and may unfold quickly, it is possible that politicians will need to take rapid decisions, and we hope that our report can guide sensible and responsible decision-making.”