No 10 tried to play down the significance of the meeting between the close ally and opposition leader – saying it was “not unusual”.
But the talks – reportedly requested by Sir Keir agreed by Mr Macron – break with the tradition which sees foreign leaders keep clear of meeting UK opposition party chiefs.
It amounts to a change in protocol for the French president, who generally only holds talks with opposition politicians from his own centrist persuasion – such as those from the European Renew group.
The PR coup for Sir Keir will see him accompanied by his new chief of staff Sue Gray, along with shadow chancellor Rachel Reeves and shadow foreign secretary David Lammy, according to Politico.
A source at the Elysée told The Telegraph that Mr Macrons had agreed to the meeting to allow the Labour leader to “test” reactions to his policies on Brexit and other areas.
“We’re not from the same political family, we are not Socialists, but … the strategic nature of the bilateral relationship justifies us receiving him and this desire to come and test the major policies with us that he may put into his electoral programme,” the source.
“What is our vision of UK-France relations tomorrow? What do we do with Brexit? What are we doing with the European Union? … Does he want to renegotiate the Withdrawal Agreement, and how does he globally see relations with Europeans tomorrow?
A spokesman for Sir Keir did not deny reports the meeting was planned for early next week, but said did not have “anything to confirm” yet.
Rishi Sunak’s official spokesman said: “It is not unusual for opposition leaders to have met world leaders in the past,” before highlighting the “strength” of the PM’s relationship with the French.
The PM has been able to forge closer ties with France after the relationship reached a low ebb under Boris Johnson. Cordial meetings with Mr Sunak and Mr Macron saw the connection dubbed “le bromance”.
The UK agreed to help fund a new detention centre in northern France, 500 extra officers and more drones as part of a £500m deal to stop small boat crossings after the pair met in Paris in March.
The meeting with Sir Keir set to take place on Tuesday happens shortly before King Charles carries out a state visit of France. Mr Macron is said to have told Mr Sunak that he would welcome Sir Keir when they met at the G20 last week.
Catherine Haddon of the Institute for Government think tank said it was “not abnormal” for opposition leaders to meet foreign leaders, though she warned it could look “presumptive”.
She said: “Starmer and his team clearly feel it works for them, and France is an important country to have a relationship with. But still, what is interesting in this is not that Starmer is doing this, but that Macron is.”
It comes as Mr Starmer prepares to meet Europol officials at the Hague on Thursday as he tries to highlight Labour’s proposals to tackle small boat crossings by asylum seekers.
The Labour leader is expected to discuss his plans to create a new cross-border cell at the National Crime Agency (NCA). The party also wants to send officers to work with border officials of “upstream” countries to break people-smuggling gangs.
Labour is also proposing more caseworkers at the Home Office to clear the asylum claim backlog, costing £6m a day for hotel and other accommodation.
Asked why Labour did not support the Rwanda deportation plan, a spokesman for Mr Starmer pointed to the costs and said money would be “much better” spent investing in the NCA.
The Labour leader will also travel to Montreal in Canada later this week for a conference of centre-left leaders to urged them to back cooperation on people trafficking.