US Election 2020: Trump 'not conceding' to Biden
Trump said in a statement that "our campaign will start prosecuting our case in court to ensure election laws are fully upheld and the rightful winner is seated".
Trump was at his Virginia golf course when the presidential race was called for Biden on Saturday.
"We all know why Joe Biden is rushing to falsely pose as the winner, and why his media allies are trying so hard to help him: they don't want the truth to be exposed," Trump said.
"The simple fact is this election is far from over."
Trump had falsely declared himself the winner of the election earlier on Saturday, when vote counting was unfinished.
He has since alleged irregularities in counting, but has not presented any evidence of election fraud.
Near complete results issued by each state showed an insurmountable lead for Biden, allowing network news channels to call the overall result, as they do every election.
In recent weeks, Trump has alleged - without evidence - widespread fraud and misconduct in the election.
His comments have drawn bipartisan rebuke from election officials and lawmakers as dangerous attempts to undermine public confidence in the vote.
Biden clinched his victory with a win in Pennsylvania, the state where he was born.