He added that it would remove unnecessary barriers to trade, as well as strengthen security cooperation with European allies.
Sir Keir campaigned to remain in the EU during the 2016 referendum, before serving as Brexit secretary in the shadow of Jeremy Corbyn and supporting a second referendum.
But his speech on Monday was his strongest signal so far that he wants to sideline the party from any association with the Remain camp.
Last month, a shadow minister said Labor would at least try to rejoin the EU’s single market and a customs union once in power.
Anna McMorrin, shadow justice minister, told supporters that Labor should renegotiate the current agreement with the EU, and suggested it could be a way to unite fully.
Speaking to the Labor in Communication group, Ms McMorrin said when asked if the UK could return to the single market: “I hope so, I really hope so.”
But then he returned to his comments and said: “Labor policy on Brexit is clear. We have left the EU, Labor voted in favor of the agreement. It is now up to all of us to make it work.”
David Lammy, the shadow foreign secretary and another potential candidate for leadership, has also supported the reworking of ties with Brussels.
“Of course, if we were in government, there are aspects of our relationship with the European Union that need to be determined,” he told LBC last month.