Senior political allies of Emmanuel Macron have lined up to warn against complacency in France’s presidential race, saying the incumbent is not certain to win despite polls indicating his lead over his far-right challenger, Marine Le Pen, is widening. “The game isn’t done and dusted and we certainly cannot draw the conclusion … that this election is already decided,” the French prime minister, Jean Castex, told French radio, five days before Sunday’s second round runoff. “We have to convince the French that Emmanuel Macron’s programmes are the best for France and for them. Emmanuel Macron and Marine Le Pen are being put on the same level, but there are enormous differences between them.” Castex’s immediate predecessor, Édouard Philippe, now the mayor of the northern city of Le Havre and, according to a recent survey, France’s most popular politician, also said nothing should be taken for granted given the many election “unknowns” – most particularly a possible low turnout. Philippe told Le Figaro that France’s “Republican front”, the alliance of centre-right and centre-left voters that has so far excluded the far right from power, was manifestly “no longer a natural reflex, doubtless due to fatigue”. François Bayrou, another heavyweight Macron backer, told La République des Pyrénées that “at this stage either candidate could win. Everything is possible. We have all seen voters make choices historians later consider crazy.” Macron himself made clear on Monday night that he did not consider the election already won, recalling the political earthquakes of 2016 when British voters chose to leave the EU and the US electorate put Donald Trump in the White House. “Think back to what British citizens were saying hours before the Brexit referendum, or in the US before the Trump vote came: ‘I’m not voting. What’s the point?’” Macron told the TV show C à vous. “I can tell you, the next day they regretted it. If you want to avoid the unthinkable or something that revolts you, choose for yourself.” Polls suggest that since the first-round vote on 10 April, increased scrutiny of Le Pen’s manifesto – in particular her proposals for a “national preference” in jobs, welfare and housing, which experts have said would violate French and EU equality laws, and for the economy, described by economists as dangerous – have slowed her progress. Macron’s projected lead for Sunday’s decisive second-round vote now averages eight or nine percentage points across all polls, with the latest, published on Tuesday, suggesting the gap has widened from eight to 12 points since Friday. The French finance minister, Bruno Le Maire, told Europe 1 radio on Tuesday that Le Pen, who has long voiced admiration for Russia and accepted loans from Russian banks, would “hand France’s sovereignty to Vladimir Putin” if she was elected. “I heard Marine Le Pen’s international policy proposals … This is the end of French sovereignty,” Le Maire said, adding that her victory would lead to an alliance with Putin, a lack of Nato protection and cutting ties with Germany. Castex said his government would tender its resignation if Macron was re-elected president, in an effort to provide a “new impetus” for the president’s centrist La République En Marche party in the run-up to parliamentary elections in June. The vote in June will be a further major test of Macron’s popularity, and renewing his majority will be essential if he is to pursue his reformist agenda, including a controversial overhaul of the pensions system. Macron and Le Pen will face off on Wednesday evening during the live second-round TV debate, traditionally a key moment in French presidential elections, which could prove critical for winning over millions of voters, in particular on the left. Le Pen was reportedly rehearsing with her closest aides on Tuesday in an effort to avoid the disastrous performance that was widely seen as having precipitated her defeat in 2017.