The Ashton-under-Lyne MP is runaway favourite to be named Tom Watson’s successor as deputy leader on 4 April.
Labour have cancelled a special conference planned to unveil the new leader and deputy leader team because of the outbreak, planning a “scaled back event” on the same date instead.
The timing means that Ms Rayner - currently Jeremy Corbyn’s shadow education secretary - is likely to still be self-isolating when the result is announced.
In a message on Twitter, Ms Rayner, 39, said her symptoms began on Thursday evening and had got gradually worse.
The battle to replace Jeremy Corbyn as Labour leader
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She said: “I am now in self isolation as l have the symptoms of coronavirus. They started last night and have gradually got worse.
“I will be a bit quieter than usual but will still be keeping in touch with people as best as l can.
“ Keep safe everybody and remember - Stay Home, Save Lives.”
Messages of sympathy and support immediately began flooding in, with former leadership candidate Jess Phillips telling Rayner "Get well bab" and Greater Manchester mayor Andy Burnham telling her "Take care Ange".
Voting closes in the ballots for leader and deputy leader on 2 April.
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