Valls headed to the heart of the British financial sector in London on Monday where he gave a speech before the country’s top bankers after meeting his UK counterpart David Cameron.
The French PM made the trip, as he did last month to Germany, in an effort to convince UK financiers that France is putting into place reforms that will cut its spending and debt as well spark economic growth.
When French president François Hollande was elected in 2012 he told France that his "real enemy was the world of finance".
But Hollande has since had a change of heart and both he and Valls have been at pains to stress the Socialist administration has the interests of companies at heart.
In his speech, at the Guildhall in London's banking district known as The City, Valls told the audience in English that his government was "pro-business".
In the rest of speech, which was given in French, he also warned that Britain would "lose a lot" if it were to leave the European Union.
"Britain, and in particular the City, would lose a lot if it were to turn its back on Europe," Valls told executives.
"France wants Britain to stay in the European Union," he said, calling also for joint action to reform Europe and make it "more intelligible to bring it closer to the people".
"There is a choice that we can and should make together: reform Europe to put growth, competitiveness and jobs back at the heart of its priorities," he said.
After a leading British businessman dismissed France as "finished" last week, Valls told bankers he intends to challenge his government to restore France as the most powerful economy in Europe.
"France has many positives," he said. "In particular its youth and its demography.
"Our first challenge is to restore the competitiveness of our businesses," he added.
The French leader also joked that the sight of a Socialist French PM visiting London's famous financial district represented "a revolution".
"A French prime minister in the City is an event. A Socialist French prime minister in the City is a revolution!," he joked.
He also delivered "some bad news for London", telling bankers that shops in Paris would soon be open on Sundays, as they are in the UK capital.
And in what might have raised a few eyebrows among financiers Valls also said that France had to cut taxes "which weight heavy on households".
His speech comes after the head of high-end British retailer John Lewis told a group of young entrepreneurs last Thursday that “France is finished” and “sclerotic, hopeless and downbeat.” He later apologized but it was just the latest example of French bashing from a British source.
Lest we forget, David Cameron himself famously said in 2012 he'd "roll out the red carpet" for French companies looking to escape a 75 percent tax rate on top salaries.
Political jibes aside, the French and British a great deal of business together. Last year alone British concerns made 42 investments in France that created some 2,500 jobs, 17 percent increase over the previous year, French economic publication Les Echoes reported.
And Britain was the fourth largest overall investor in France behind the United States, Germany and Italy. It's also worth noting that British investors have more money tied up in France than anywhere else in Europe.
Notable examples include the hugely successful roll out of low-cost British retailer Primark and the continued presence of Marks & Spencer department stores in France. That said the British economy is expected to grow 3.2 in 2015, while there's debate over whether French growth will even hit one percent.
Valls's mission tol London echoed a visit to Germany last month when he made efforts to reassure Berlin that i twas up to France to get its own house in order.
"There is a clear commitment by the prime minister to persuade all of our European partners of what is happening in France," an unnamed French diplomatic source told AFP.
"We see the European press and the attention focused on us. They are asking 'what is happening in France?', hence the need for us to carry out these visits," she added.
If negotiations with fiscal hawk Merkel were tough, they threaten to be even harder with Cameron, who is currently trying to appease the eurosceptic wing of his Conservative Party and fend off the anti-European UK Independence Party ahead of next year's general election.
He has already promised to hold an in-out referendum in 2017 on Britain's membership of the European Union if his party wins the election.
The meeting comes amid criticism of France for not doing enough to reduce its budget deficit next year and suggestions made by an EU source to AFP that Brussels is set to reject the new Valls government.
On Monday afternoon, Valls is likely to find a more sympathetic ear when he meets Ed Miliband, leader of the centre-left opposition Labour Party.