The Trade Unionist and Socialist Coalition (TUSC), which fielded more than 130 candidates at the 2015 general election, will not contest any seats on 8 June.
The hard-left group, founded by former RMT general secretary Bob Crow in 2010 and supported by the Socialist Party of England and Wales (England and Wales), will instead back Jeremy Corbyn and Labour at the ballot box.
The Socialist Party is better known as an offshoot of the Militant Tendency, the Marxist entryist group which infiltrated Labour in the 1970s and 1980s.
TUSC national chair Dave Nellist, a former Labour MP in Coventry and Socialist Party member, said the organisation wanted to put a socialist in Number 10.
"We know that getting 326 or more Labour MPs elected on 8 June wouldn't stop the Blairites continuing to plot against Jeremy. They did it last summer and, if they feel confident enough, they could do it again this summer," Nellist said.
"They don't support the socialist policies of taxing the rich and public ownership that are needed to overturn the 'rigged system' and they will do what they can to resist them.
"But defeating the Tory government would be seen as such a victory for Jeremy Corbyn it would inspire and give confidence to millions that a different society is possible.
"It would create the chance to build the mass support base that Jeremy will need against the capitalist establishment to implement his policies and we will work determinedly towards that end."
The RMT, which is not affiliated to Labour unlike fellow transport unions such as TSSA and Aslef, described Corbyn as the "genuine article".
"In contrast to Jeremy Corbyn's Labour Party, Theresa May is a hard right politician who is seeking a mandate to carry out the agenda of the big business establishment," said Mick Cash, the general secretary of the RMT.
"That means more cuts to our public services and driving down pay and job security through shredding employment and trade union protections."
A leaked copy of Labour's general election manifesto shows that the party plans to propose re-nationalising the UK's railways, energy firms and the Royal Mail. The latest opinion from YouGov, of more than 1,600 voters between 9 and 10 May, put the Conservatives 16 points ahead of Labour (46% versus 30%).