Jeremy Corbyn has declined to say that he would honour the terms of a Brexit deal struck by Theresa May, if he succeeded her at Number 10.
He also defended dismissing "no deal" as a viable outcome to the Brexit negotiation, saying it would be "catastrophic for manufacturing".
He told Sky News: "It depends what the deal is and what is achieved, if it was before 2019 then obviously we'd have to look at it very carefully.
"We've demanded all along that parliament be involved in this and the Government give regular votable updates on the negotiations. The problem is there's been no votable update; there's been no negotiations."
Mr Corbyn was in Brussels at a pre-summit of centre-left European leaders and MEPs in Brussels, meeting three EU Prime Ministers and chief EU Brexit negotiator Michel Barnier.
The Labour leader was met with a rapturous applause by the meeting of European left-wing politicians.
The end of his speech was attended by the Prime Ministers of Italy, Portugal, Sweden, Malta and by European Commissioners, as they joined him on stage for the family photo of the Party of European Socialists pre-Summit meeting.
He told the audience that "the British Labour Party does not see anyone in Europe as our enemies – you are our friends, colleagues & comrades".
He told Sky News: "We're here to discuss with other prime ministers and Mr Barnier the lack of progress with Mrs May and say that we want jobs and services for Britain
"We want a serious and constructive relationship with Europe and we recognise the result of the referendum and we have to move forward."
Asked if he was offering a back channel for the EU to easier Brexit negotiations with them he said: "We've said all along our priority is economic questions protecting jobs and employment.
"No deal means go off the cliff edge. What happens to jobs in Sunderland? Jobs in Bridgend?
"What happens to jobs where there is no export to Europe? What happens to the food processing industry?"
He said that no deal would be catastrophic for manufacturing, and that he was not talking down British prospects.Let's