Jeremy Corbyn has said Diane Abbott has done a "good job" in the shadow cabinet, but has refused to commit to appointing her as home secretary if Labour wins.
Ms Abbott pulled out of two election events on Tuesday due to illness, hours after a difficult interview on Sky News with Dermot Murnaghan.
She was unable to answer questions on recommendations made in a key counter-terrorism review about how best to prevent terror attacks in London.
In a series of vague answers after long pauses, it became clear Ms Abbott was unable to recall details of the review by Labour's Lord Harris.
The Conservatives have repeatedly criticised the shadow home secretary, seeing her as a weak link in Labour's top team.
Speaking to reporters on Labour's battle bus in Telford, Shropshire, Mr Corbyn defended his close ally - but refused to commit to appointing her as home secretary if he wins Thursday's General Election.
Asked if he had confidence in Ms Abbott, Mr Corbyn said: "Our shadow home secretary has done a good job, she's not well today and is therefore not on the campaign today."
Asked if her media appearances had damaged Labour's campaign, Mr Corbyn said: "Diane has been a great advocate for the Labour Party and a great advocate for young women trying to achieve in politics, and I think we should recognise she has received an awful lot of very unfair criticism and abuse in the past, she's not well at the moment and she's taking a break for the campaign."
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