"There was no intention, nor did the motion rule out the option of remain".
"Everybody recognizes the talks are going badly and it looks as though we're heading for a bad deal or even no deal", he told BBC TV.
"We don't want to face that situation and we have got a duty to do something to stop it and that's why the option of a public vote is important as something that may have to happen when we get to that stage".
The commitment was seemingly ad-libbed on stage by Starmer, as it was not included in the released text of his speech.
Delegates at the party's conference are today expected to back a plan that could lead to a fresh referendum.
Motorists will need to carry a "Green Card" proving they have third party motor insurance cover in order to drive in the European Union after a no-deal Brexit. May has a working majority of just 13 in the 650-seat parliament and a former junior minister said this month as many as 80 of her own lawmakers were prepared to vote against a Brexit deal based on the Chequers proposals.
Ian Wright, chief executive of the Food and Drink Federation, said the "official confirmation of just how bad this scenario would be is bound to encourage businesses and shoppers to consider - now - stockpiling, buying ahead, hedging currency risk, procuring additional warehousing, relocating production to the European Union, and other practical measures to secure supply".
Flights between the United Kingdom and 17 non-EU countries, such as the US, Canada, Switzerland and Iceland, now operate due to the United Kingdom being a member of the EU.
New documents have laid bare the "deeply unsettling" possible consequences of a no-deal Brexit, suggesting there could be chaos at ports, disruption to food supplies and increased business costs.
Many business chiefs and investors fear politics could scupper an agreement, thrusting the world's fifth largest economy into a "no-deal" Brexit that they say would spook financial markets and silt up the arteries of trade.
Labour's position leaves the prime minister brutally exposed to a rebellion by restive Tory backbenchers, with fewer than a dozen able to fracture her fragile control of the Commons in the upcoming vote.