The election campaign is raising "profound questions" about the legitimacy of the voting system that means Britain must at least "explore" a form of proportional representation, the former cabinet secretary has said.
Lord Gus O'Donnell, who presided over the crucial 2010 coalition negotiations, had previously warned the election would throw up questions of "democratic legitimacy" amid the increasing popularity of smaller parties, particularly the SNP.
But he told The Huffington Post UK that issues such as the effectiveness of First Past The Post (FPTP) and the size of constituencies needed to be addressed "away from the party politics" of the election campaign.
"I think we should certainly explore proportional systems, more proportional systems, yes, I do," O'Donnell said, when asked if Britain should use Alternative Voting (AV).
He added that AV and Scottish independence - both of which were rejected in referenda during this parliament, would continue to be "profound questions" in the next one.
He said: "A number of issues have arisen during from this election that are worth thinking about away from elections. So early in the next parliament, we should look at a principles-based approach to say, TV debates.
"We should have a think about, do we think our voting system and our boundaries are delivering a good democratic outcome?
"Those are the kind of things where you want to do it an evidence-based way, away from party politics and general elections and just coldly, try and have a principles-based approach."
He added it was "ironic" that FPTP had produced a coalition and may produce another, when it was popular because it was meant to produce single party governments, while the Scottish parliament system, which uses a form or proportional representation designed to produce coalitions, has produced a single party government.
"There's a lot of irony about our system," he added.
He added the issue over whether Scottish MPs could vote on matters affecting only England should also be looked at.
O'Donnell, who is now a cross-bench peer, said it looked like Ukip and the Green Party would receive more votes than the SNP but the number of seats won would not reflect this.
"The predictions are Greens getting one seat, Ukip getting between one and five and SNP, with 4% of the vote, getting somewhere between 35 and 55," he added.
"It'll be quite likely that England will be Conservative and Scotland will be, not just not Conservative, but will be SNP. That's something new.
"If there's a Conservative prime minister, then the SNP will be repeating their message there were using during the referendum - 'one of the things about independence is, you get the government you vote for'.
"They'll be saying, 'so here we are in Scotland, who did we vote for? Well it wasn't that we voted Labour. We voted SNP and we got Conservatives and curiously enough, there are somewhere between zero and two Conservative MPs [in Scotland]'."
When asked if a coalition or a minority government would be better in the likely event of a hung parliament, O'Donnell said people should not underestimate minority governments that have supply and confidence deals with smaller parties, where the smaller party votes with the government on supply bills and any motions of no confidence.
He said: "The main issue is getting something which can last through for a five-year parliament. I think people have underestimated the supply and confidence deals.
"They can be quite strong. It will really depend, it's very hard to say."
O'Donnell made a brief appearance in Channel 4's Coalition, which dramatised the days after the 2010 election and negotiations that created the Tory-Liberal Democrat government. He was played by actor David Annen.
The fictional O'Donnell was depicted trying to prevent Labour from returning to Downing Street after the election result is declared and the party is negotiating to remain in power.
"It was quite brief but it was fine," O'Donnell said of his screen time. "I wasn't George Clooney but, y'know, there you go. There were lots of little details that one could quibble with but, I mean, broadly, it was pretty reasonable."
When asked which details he would quibble, he said: "I'm not gonna get into that."