"A member of the Labour national executive committee told the Sunday Times: "It is pretty clear that what is happening amounts to infiltration of the Labour party."
The Communist party has urged its supporters to vote for Jeremy Corbyn in the leadership race.
Labour MP John Mann told the Sunday Times the situation was "totally out of control" and called on acting leader Harriet Harman to suspend the process for admitting new members while checks can be carried out.
"It is becoming a farce with longstanding members... in danger of getting trumped by people who have opposed the Labour party and want to break it up," he said.
John Cryer, who chairs the Parliamentary Labour Party, told the Mail on Sunday members of the TUSC have been joining as registered supporters for £3, which gives them the right to vote in the leader and deputy leader elections.
The TUSC was formed in 2010 with the help of the Socialist Party, which was the successor of Militant Tendency.
"These people were thrown out of the Labour Party and should not be allowed to vote," Mr Cryer said to the newspaper.
Former minister Lord Hutton added that it would be a "travesty" if members of Militant Tendency were able to influence the election."
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