Former attorney general Jeff Sessions will face a runoff to regain his old Senate seat after failing to win a majority in the Alabama Republican primary contest.

Mr Sessions had won 32.6 per cent with more than half of the votes counted in the race. His closest rival was Auburn University football coach Tommy Tuberville, who won 32.2 per cent.

The pair will likely face each other in a March run-off, the winner of which will go up against Democrat Senator Doug Jones, who won a special election in 2017.

He will go into the runoff without the support of his former boss, Donald Trump, who called him the “biggest mistake” of his presidency.

Mr Sessions left his Senate seat to join the Trump administration, but fell out of favour with the president after recusing himself from the investigation into Russian meddling in the US election.

Mr Jones won in the deeply red state three years ago after his Republican rival Roy Moore was accused of sexual misconduct by several women, some of whom were teenagers.

President Trump’s approval rating is in the low 60s in Alabama, and the Senate seat is now a key target for Republicans.