The ex-Liberal deputy also says a shake-up of the way MEPs are elected every five years would resolve the problem of how to distribute the 73 seats vacated by British deputies when the UK pulls out of the EU.
Duff says transnational lists for pan-European seats, based on a scheme known as the 'Cambridge Compromise' (CamCom) is the best solution.
This, he insists, would also boost turnout in the Euro elections which has traditionally fallen each time since the first direct elections in 1979.
Federalist members, including Duff, in the Parliament have made several efforts in the past to introduce transnational lists for pan-European seats, but until lately these efforts have stalled in the face of divisions within the mainstream groups and outright hostility from British MEPs
Duff asks, "Our main question today is: do the circumstances exist to introduce European lists for the Parliament in time for the May 2019 elections? Brexit now removes at least one major obstacle to the political reform of the system."
Duff believes President Macron will add "considerable weight" in Council to the cause he has pushed for years and which is already being strongly advanced by the Italian Europe minister, Sandro Gozi and supported by Belgium.
Duff told this website, "The big game-changer, of course, is that we now have 73 ex-British seats to play with."
His proposal is based on the so-called CamCom scheme.
In 2011 the European Parliament's Constitutional Affairs Committee gathered Europe's specialist mathematicians and political scientists in Cambridge to devise a formula. Now known as the Cambridge Compromise (CamCom), this formula gives a base of five seats to all states supplemented by a number of seats relative to its population, rounded upwards.
Duff said, "What this exercise has done is to show that it is the middling sized states that are at the moment over-represented in the parliament, most notably Hungary, and that both smaller states and the largest (Spain and France in particular) are under-represented."
He said, "CamCom is fair, simple, straightforward and transparent and meets the European Council's requirement of facilitating technical adjustments between each parliamentary mandate to deal with population shifts and the accession or secession of member states.
"CamCom can be introduced to guarantee digressive proportionality without causing any member state, even Hungary, to lose a seat. France and Spain could be offered a sweetener of a number of extra 'national' seats."
He went on, "But the bulk of the ex-British seats - say 50 of them - could easily be filled from the first transnational list for the first pan-European constituency."
Duff, a constitutional expert, says the time for such a radical reform is now.
"The introduction of European lists would at a stroke Europeanise the European elections and re-invent the Parliament. It would put the Parliament back on the right side of EU law in so far as its composition would respect the principle of digressive proportionality.
"It would install a genuinely uniform element in Parliament's electoral procedure and properly reflect the function of the MEP as representative of all the Union's citizens. European lists coupled with CamCom would settle the controversy over seat apportionment."
He explained further how it would all work; "The EU electors in 2019 would be offered two ballot papers on entering the polling station: one for their traditional national or regional constituency, the other for the European."
Duff, now a visiting fellow at the European Policy Centre, added, "Many would be amazed; some would be confounded; but most would enjoy their first material prize from the privilege they enjoy as EU citizens. European democracy would be refreshed."
Emmanuel Macron, he said, is the first European leader to be elected on a platform that embraces the electoral reform of the Parliament.
Duff, who lost his seat in the last European elections, said, "Macron supports proposals to create a pan-European constituency for the election of a certain number of MEPs from transnational lists.
"Those lists would be the creature of the EU-level political parties which would, for the first time, be given a real role in the conduct of the election campaign for the Parliament.
"Macron is convinced that Parliament needs greater legitimacy if fiscal integration is to deepen within the eurozone, and that such legitimacy will only come if the relationship between the Parliament and the electorate gets to be closer and more direct than it is at present.
"Until now European elections in France have been strangely alien affairs: neither national lists nor regional lists have sparked genuine interest among voters, and once elected French MEPs have been left marooned by their national political parties and distant from the voter."
Recently, Italy tabled similar plans for a transnational list for the 73 UK seats the Parliament.
European Green Party co-Chair Monica Frassoni, a former MEP, said, "The Italian proposal comes at the right time to relaunch the debate on a fully fledged European wide political competition, something the Greens have been expecting and hoping for a long time.
"Transnational lists are linked to a vision of a more democratic, open and inclusive Europe, as they open a debate among political forces and candidates on the different options for the EU, beyond national priorities and dynamics."