The shadow international trade secretary put a recent series of car crash interviews from the politician down to her illness after it was announced Lyn Brown would temporarily replace her as shadow home secretary.
He insisted he did not know what her medical condition was confirmed it was a “serious long-term illness”.
He told talkRADIO’s Julia Hartley-Brewer: “Diane is clearly not well and I understand that it’s a condition which has been diagnosed and is long-term.
“And I think anybody who’s seen her in the past couple of weeks would realise that she was showing that she was not well in the way in which she had been operating.
Barry Gardiner said Diane Abbott's health had affected her performance during recent interviews
I think anybody who’s seen her in the past couple of weeks would realise that she was showing that she was not well in the way in which she had been operating
“I’m sorry for her obviously, that she’s got this condition, and clearly it’s now a matter for her to get properly diagnosed, properly treated and I wish her very well.”
He said the reasons behind the Jeremy Corbyn ally’s performances during TV interviews had “become clear”.
“Everybody is aware that Diane did not perform well on a couple of programmes but I think what we didn’t know was why and I think that has now become clear,” he said.
But the presenter refuted the claim before Mr Gardiner insisted: “We haven’t been told what the condition is but my understanding is that she’s been diagnosed.”
Diane Abbott's 10 worst gaffes Wed, June 7, 2017
Diane Abbott has a long history of saying the wrong thing – here 10 of her best (or worst)
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In an interview on LBC radio, Abbott suggested that hiring 10,000 police officers would cost £300,000. Meaning each officer would earn £30 a year
Diane Abbott at the Labour Party's manifesto launch
Hartley-Brewer then laid into the Labour politician’s defence of Ms Abbott.
She said: “Are you implying that she has some sort of long-term illness that has affected her ability to remember things like police numbers?
“I’m not being jokey, genuinely, are you talking about something that means that she is tired, something that means that her brain is not working properly, are we talking about cancer – what are we talking about?”
Mr Gardiner wished his colleague well once more and insisted he did not know her medical condition but confirmed Ms Abbott had been “coming to terms” with it.
A Labour spokesman said: "Jeremy Corbyn has asked Lyn Brown to stand in for Diane Abbott as Shadow Home Secretary for the period of her ill health."