Jeremy Corbyn will be automatically included on the ballot in Labour's leadership contest, the party's National Executive Committee has ruled.
Unions said party rules were clear the incumbent who is challenged must be allowed to stand but his opponents said he needed support of 51 MPs or MEPs.
The leadership contest was sparked after former shadow minister Angela Eagle challenged Mr Corbyn.
Ms Eagle said she welcomed the contest ahead and was "determined to win it".
The secret NEC vote went 18-14 in the Labour leader's favour following hours of talks.
A Labour Party spokesman said: "The NEC has agreed that as the incumbent leader Jeremy Corbyn will go forward onto the ballot without requiring nominations from the Parliamentary Labour Party and the European Parliamentary Labour Party.
"All other leadership candidates will require nominations from twenty percent of the PLP and EPLP."
Shadow chancellor John McDonnell, a key ally of Mr Corbyn, said on Twitter: "Jeremy on the ballot. Democracy prevails. We will use leadership election to sign up even more members and prepare ground for general election."