Prime Minister Malcolm Turnbull's same-sex marriage plebiscite plan appears to be in tatters after Opposition Leader Bill Shorten sent his strongest signal yet that Labor will block it amid growing fears a popular vote will fail and set back the cause.

In exclusive comments to Fairfax Media, Mr Shorten has hardened his criticism of the plebiscite plan in a clear indication Labor is preparing to announce it will not support the Coalition's enabling legislation.

"I'm worried Malcolm Turnbull will just stuff it up," Mr Shorten said. "He stuffed up the republic referendum, he stuffed up the NBN and he stuffed up Senate reforms when he promised to fix it."

Mr Shorten's leadership group discussed the issue behind closed doors this week. While it's understood they did not reach a final decision – and the issue may still be put to the full Labor Party room – they are increasingly inclined to vote it down.