Emily Thornberry has confirmed that she will enter the race to replace Jeremy Corbyn as Labour leader in January.
The shadow foreign secretary is the first candidate to formally announce her bid to replace Mr Corbyn, as the party tries to recover from its worst general election result since 1935.
Other possible candidates include shadow Brexit secretary Sir Keir Starmer and Wigan MP Lisa Nandy, who have both signalled they are considering a bid.
Key Corbyn ally Rebecca Long-Bailey is seen as the choice of the current leadership, with shadow chancellor John McDonnell giving her his backing.
Ms Thornberry cited her experience in confronting Boris Johnson across the dispatch box when he was Foreign Secretary as she announced her plans to run.
She said the first question for any leadership hopeful should be: "What is your plan for taking on Johnson over the next five years?", claiming she had previously "pummelled him every week".
Ms Thornberry, a former barrister, said Labour should fight on pledges for the elderly, on housing, unemployment and child poverty.
She was highly critical of the Labour leadership for backing Boris Johnson's call for an election on Brexit.
Mr Corbyn announced he would stand down from his role as party leader last week.
He assured colleagues he would stay on until a new leader was elected to ensure "the smoothest possible transition".
talkRADIO: Listen live