Diane Abbott says she will be back in action soon after it was announced she was "taking a break" from campaigning.
The shadow home secretary tweeted: "Touched by all the messages of support. Still standing! Will rejoin the fray soon. Vote Labour!"
Labour leader Jeremy Corbyn announced earlier that Ms Abbott "was not well" and was taking a break.
Shadow minister Lyn Brown is to stand in for Ms Abbott, who pulled out of two media appearances on Tuesday.
Asked how long Ms Abbott would be taking a break, Mr Corbyn said: "I'll be talking to her later on today."
He said: "Of course Diane is somebody that works extremely hard and represents her community very well and I have to say has received totally unfair levels of attack and abuse not just recently - over many years."
BBC political editor Laura Kuenssberg said Ms Abbott's break would be "indefinite" and political correspondent Iain Watson said friends of Ms Abbott had told him she was moved without consultation, not at her own request.
'It has now become clear'
Labour's international trade spokesman Barry Gardiner said he had been told by Labour Party officials that Ms Abbott was suffering from a long-term illness.
"Diane is clearly not well and I understand that it is a condition which has been diagnosed and is long-term," Mr Gardiner told TalkRadio.
"I think anybody who has 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."
He added: "Everybody is aware that Diane didn't perform well in a couple of programmes, but what we didn't know was why and I think that has now become clear."
Mr Gardiner later told the Press Association it was a matter for Ms Abbott to decide what details of her health should be made public.
Ms Abbott had been scheduled to appear on a special election edition of Radio 4's Woman's Hour alongside Home Secretary Amber Rudd and other political rivals on Tuesday.
But shortly before transmission, it was announced that shadow foreign secretary Emily Thornberry would take her place.
Ms Abbott had downplayed claims Labour wanted her to adopt a lower profile after several faltering performances.
During an appearance on Sky News on Monday, Ms Abbott rejected suggestions that the party leadership regarded her as a liability after an LBC interview earlier in the campaign when she failed to put an accurate cost on the party's plans to fund 10,000 new police officers.
She said she was appearing regularly in the media and it would be "strange" if she was not doing so at a time of heightened concern about security and policing after the terror attacks in London and Manchester.
Lyn Brown, 57, was first appointed to Labour's shadow home affairs team when Mr Corbyn named his first frontbench team in 2015.
But she was among a slew of MPs who resigned the following year following a vote of no confidence in his leadership. She said then that Labour should seek a new leader "for the good of the party and the country".
She rejoined the front bench three months later as minister for policing. MP for West Ham since 2005, she is fighting Thursday's election in the ultra-safe east London seat, which she held with a majority of almost 28,000 in 2015.
She was a Labour whip under Gordon Brown and Ed Miliband, before being appointed shadow minister for communities and local government in 2013 and then shadow home office minister two years later.
The Conservatives said Labour were "hiding" Ms Abbott away from voters as she was "not trusted" by Mr Corbyn, a close ally and friend of hers, and other colleagues.