Speaking at the Fabian conference on Saturday morning, Starmer told an audience of Labour Party members: "We are now at the third phase of our policy. Make no mistake about it."
The "third phase" refers to Labour policy of considering the option of a "public vote" on Brexit if the party is unable to bring down Theresa May's government and trigger a snap general election.
Labour leader Jeremy Corbyn tried to bring down the prime minister this week by laying a vote of no confidence in her government after her Brexit deal was rejected by a record-breaking House of Commons majority.
Starmer said on Saturday that while triggering a general election "always will be" Labour's desire, in the short-term the party has "just two remaining options left" on Brexit.
The first is to force May into accepting Labour's alternate Brexit deal of a "close economic relationship" with the EU based on permanent customs union, strong ties with the single market, and robust workplace and environmental standards.
However, if Labour cannot achieve this, then the party will honour its commitment to backing a new referendum, Starmer said.
"Our conference motion states that: 'If we cannot get a general election Labour must support all options remaining on the table, including campaigning for a public vote.'
"That is a very important commitment. Its a commitment to you, our members and our movement. And it is one we will keep," he said on Saturday morning.
"I know there is significant support for this in our membership In many trade unions. Among a number of Labour MPs.. in this city and most likely in this hall. As I set out in Liverpool, this has to be an option for Labour."
As Business Insider has reported , Corbyn's office is reluctant to commit to a new referendum because it believes to do so would alienate Leave-voters in seats that the party must in order to win the next general election.
However, the Labour leader is under growing pressure to back a new referendum or what campaigners call a People's Vote from at least 71 Labour MPs and the party's strongly pro-EU membership.
In his speech on Saturday, Starmer repeated his prediction in stronger terms that Brexit will be delayed.
"We also need to recognise that whichever of these options we pursue the March 29 deadline looks increasingly unlikely to be met," the Labour MP said.
"Even if the Prime Ministers deal had been passed on Tuesday, there is a huge raft of legislation the Government would still need to pass.
A complex Implementation Bill. An Immigration Bill. Agriculture and Fisheries Bills. The Trade Bill. A Healthcare Bill. A Financial Services Bill.
"69 days. Six bills. Plus around 700 statutory instruments that still need to be passed. I don't think so."
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