The PM said he looked forward to working with Luiz Inacio Lula da Silva on issues “from growing the global economy to protecting the planet’s natural resources”.
Known as Lula, Brazil’s Leftist president narrowly defeated Jair Bolsonaro in an election run-off.
The far-Right incumbent had not conceded yesterday, amid fears he may contest the result.
It marks a stunning reversal in fortunes for 77-year-old Lula, who was president from 2003-10 but was jailed amid corruption claims – convictions later annulled.
He vowed to overturn Mr Bolsonaro’s legacy, such as pro-gun policies and weak rainforest protections.