In a leaked memorandum, Jeremy Browne said French authorities were seeking "disruption".
"They are in favour of the hardest Brexit," he wrote.
"They are crystal clear about their underlying objective: the weakening of Britain, the ongoing degradation of the City of London."
Mr Browne a former Liberal Democrat minister and since 2015 the City's special representative to the EU, attended talks with various French authorities earlier this month.
The memorandum, which was leaked to the Mail on Sunday, summarised for ministers Mr Browne's impressions following those talks and others in Luxembourg, where he said the tone was more co-operative.
"The meeting with the French Central Bank was the worst I have had anywhere in the EU " he wrote.
He acknowledged that the French were possibly simply taking tough negotiating positions, the "bad cop" approach, but said that "nevertheless ... France sees Britain and the City of London as adversaries, not partners."
"Every country, not unreasonably, is alive to the opportunities that Brexit provides, but the French go further, making a virtue of rejecting a partnership model with Britain and seemingly happy to see outcomes detrimental to the City of London even if Paris is not the beneficiary."
Mr Browne said the mood in France was "more giddy and more assertive" since Emmanuel Macron was elected president in May and that "French representatives [were] currently crashing conspicuously around London, making heroic relocation promises."
Several European cities, including Frankfurt, Dublin and Brussels as well as Paris are vying to attract business that may shift out of London following the UK's departure from the EU.
Financial institutions are drawing up contingency plans to open new subsidiaries within the rest of the EU to ensure they can continue to do business-as-usual post-Brexit.
Mr Browne suggested that France's adversarial tone was jarring with other EU member states, aiming for a more conciliatory approach.
"There is plenty of anxiety elsewhere in the EU about the French throwing their weight around so aggressively, but their destructive impulses are not being confined, and other EU countries that want a friendly relationship with Britain and the City of London are being marginalised," he wrote.