And he insisted a second referendum would be “far better” than the deal on the table from the Tory leader.
A five-day Commons debate on Mrs Mays Brexit deal is set to begin next week before a “meaningful vote” on December 11.
Mr Starmer told Sky News Sophy Ridge On Sunday today: “It seems to me that if the Prime Minister has lost a vote of that sort of significance then there has to be a question of confidence in the Government.
"I think it's inevitable that we will seek to move that – obviously it will depend on what actually happens in nine days, it will depend on what the response is.”
ANXIOUS: Theresa May faces a no confidence motion from Labour if she loses the Commons vote (Pic: GETTY)
“If she's lost a vote of this significance after two years of negotiation, then it is right that there should be a general election."”
Mr Starmer added: “But if she's lost a vote of this significance after two years of negotiation, then it is right that there should be a general election."
It comes as Mrs May was warned she is on course for a “historic constitutional row” unless the Government releases its full legal advice on the Brexit deal.
Furious MPs across parliament blasted ministers after they said they would only release a "full reasoned political statement" on the legal position.
WARNING: Keir Starmer says Labour is ready to launch a no confidence vote (Pic: GETTY) Related Articles
Writing in todays Sunday Telegraph, Mr Starmer insisted it was essential MPs knew the “full legal implications” before the crucial vote.
"If the full legal advice is not forthcoming, we will have no alternative but to start proceedings for contempt of Parliament – and we will work with all parties to take this forward," he said.
"If ministers stubbornly refuse to obey the order of MPs then they risk triggering a historic constitutional row.
"Although I accept the long-standing convention that Cabinet legal advice should be kept confidential, it's well-established that in exceptional circumstances that convention does not apply. And these are exceptional circumstances."
Under Commons rules, if it is ruled that a contempt had occurred the most severe punishment is expulsion from the House.
Although that is unlikely, any finding against the Government would be potentially highly damaging for Mrs May at a time when she is at her most vulnerable politically.</span>
SMILING: But will Theresa May be happy after the crunch vote on December 11? (Pic: GETTY)
CHALLENGING: Michael Gove believes the alternatives to Mrs May's deal are no deal or no Brexit (Pic: GETTY )
Conservative Party chairman Brandon Lewis insisted the Prime Minister's deal is the "only option", telling the same programme: "Plan B is plan A – it's to get this deal agreed."
And Environment Secretary Michael Gove insisted the government can win the vote on December 11 – but he acknowledged it would be “challenging”.
"I believe that we can win the argument and win the vote. I know it is challenging," he said.
“I reflected long and hard about this deal but I concluded, like lots of people, that while it is imperfect it is the right thing to do.
"One of the things that I hope people will have the chance to do over the next nine days is to recognise that we should not make the perfect the enemy of the good.
"We have got to recognise that if we don't vote for this, the alternatives are no deal or no Brexit."