He said: "It is at testing times like these that we are reminded of the values and the resolve that we share with our closest allies in Europe".
Still trying to shore up her authority after last week's election calamity injected new uncertainty into Britain's path to leaving the European Union, Ms May is sending her Brexit Secretary David Davis to Brussels on Monday to launch the process with EU negotiator Michel Barnier.
Speaking at the Elysee Presidential Palace, Macron stated that once the process to leave the 28-member bloc started, the rest of the European Union members should be collectively clear that it would be harder to reverse such a course. He said once there was sufficient progress on those, the talks would start looking at the EU's new relationship with Britain.
However, the speech was cancelled following the Grenfell Tower tragedy in West London.
"But it's not very strong and stable".
Asked if that meant no transitional arrangements, she replied: "Well, I'm extremely optimistic that we will find there is a lot we can agree on".
Olly Robbins: Senior Whitehall mandarin who will co-ordinate the officials involved in day-to-day negotiations.
The Prime Minister's legacy will be defined by whether she makes good on her promise to make a success of Brexit.
"The best way we can spend this week is to rebuild trust", rather than tackle the big hard issues right at the start, another European source said.
His initial bullish stance has been tempered with experience in Government and he has already acknowledged it could take "years and years" to bring immigration down, and suggested the United Kingdom may have to pay for access to the single market.
Echoing public opinion, five prominent industry groups called for continued Single Market access on Sunday.
After the general election fiasco, May hopes to secure the backing of the ultra-conservative Democratic Unionist Party, which would add the DUP's 10 seats to the Conservatives' 317 in the 650-member House of Commons.
"My clear view, and I believe the view of the majority of people in Britain, is that we should prioritise protecting jobs, protecting economic growth, protecting prosperity as we enter those negotiations", Hammond told reporters before a meeting of the 28 European Union finance ministers in Luxembourg.
The pound has fallen about 14 per cent against the dollar since the June 23 referendum, pushing up inflation even as the economy shows signs of slowing.
How long have they got to sort this out?
But British society has remained deeply divided about the meaning of the Brexit vote and the extent to which leaders should pull out of wide-ranging relationships that have delivered prosperity and frustration to generations of British citizens.
But the true figure could be far lower, as the 100 billion does not account for tens of billions that Britain is set to get back in shared assets and rebates.
"The more that Britain is unstable politically, the more hard it is to complete the talks on time", Grant said.
Three days after the talks begin, May is due to travel to Brussels for an European Union summit - a chance for the other 27 leaders to take stock of their negotiating partner in the sharply altered climate brought about by the dramas of the past two weeks.