Jeremy Corbyn has left the door open for Labour to campaign to leave the European Union as he addressed the party's MPs and peers for the first time as leader.
Labour "can't just give (David) Cameron a blank cheque" and any changes the PM secures "must be the right ones", the left-winger told the Parliamentary Labour Party (PLP).
Mr Corbyn arrived at the PLP to silence, rather than the traditional table thumping a newly elected leader is often greeted with, and faced some "hostile" questions during the meeting.
Shadow foreign secretary Hilary Benn earlier attempted to calm fears among many Labour MPs that Ed Miliband's successor could campaign for Britain to leave the EU in the in/out referendum expected next year.
But Mr Corbyn told MPs the Prime Minister "can't just come back with whatever", a spokeswoman said.
Asked if he ruled out campaigning to leave in all circumstances, she added: "He is saying we have to be really clear about the changes we want to see."
Mr Corbyn did not directly address the issue of military intervention in Syria, the spokeswoman said.
"On the Syria issue, he said we need to make sure we are all engaging with other countries, and with things like Isis we need to be making sure we are cutting off their arms and their supplies," she added.
Mr Corbyn, the Stop the War Coalition chairman, who is attending the Battle of Britain commemorations on Tuesday, did not directly answer questions about if he would wear a white poppy when pressed about the issue during the meeting.
"He said that people wear white poppies in remembrance of those who died," the spokeswoman said.
Mr Corbyn received "warm applause" in the meeting, which ran for more than an hour, and "quite a lot" of the questions "weren't actually hostile", she added.
The Opposition leader told the PLP that his top priorities are housing, next year's elections in Scotland and Wales, and winning the general election in 2020.
Mr Corbyn and his deputy Tom Watson will spend one day a month in Scotland ahead of the vote, a party spokesman said. Since his victory on Saturday, 28,000 people have signed up as full members of the party.