Neil Kinnock is the latest Labour heavyweight to warn the party over electing Jeremy Corbyn as leader.
Lord Kinnock, who lost two elections to the Conservatives, said he understood why people were supporting the left winger but the party had to make a decision about contesting power.
"I can see why people are angry and want to protest," he said.
"But then they have got to make a decision whether they want to be part of a Labour movement which produced a political party to seriously contest democratic power or they want to be in perpetual demonstration, which is fulfilling and noble but ultimately rarely effective."
Labour's Chuka Umunna urged his party to unite around its new leader in an apparent offer of reconciliation to left-winger Jeremy Corbyn.
Some 3,000 infiltrators have been discovered and barred from voting in the Labour leadership contest, a party source has told ITV News.
The Labour leadership candidates will meet with acting leader Harriet Harman after accusations the vote was infiltrated by opponents.