But he said was "determined" to reach an agreement ahead of the October deadline, though he said there would be "flexibility" for further negotiations.
The EU proposes that Northern Ireland stay aligned with the remaining 27 bloc members after Brexit as part of a "backstop", or insurance policy to avoid the reimposition of border checks and a risk to Irish peace gains.
After talks with the EU's chief Brexit negotiator Michel Barnier, Raab said he was "as confident as before, if not more" that there will be a deal with the European Union on Brexit. "It's essential to conclude negotiations because as I've said, without a backstop, there can be no agreement".
After meeting British Brexit minister Dominic Raab in Brussels, Barnier hailed progress on some security issues but warned there were still major differences over protected geographical indicators like champagne and stilton - and Ireland.
"We must have a detailed backstop solution which is legally binding in the Withdrawal Agreement", Mr Barnier said.
Politicians fear that a return to customs checks and police on the land frontier between Northern Ireland and the Irish Republic would revive memories of violence in the past, potentially endangering the peace.
Standing alongside Raab, Barnier said: "If you take account of the date chosen by the United Kingdom to leave, that's March 29 which is in UK law and you simply count backwards the time that you need for ratification about three months here or there then it takes you to November at the latest".
"This backstop is critical".
European Union leaders will meet on 18 October, and while agreement on a withdrawal deal is no longer seen as likely by that data, Mr Raab said that he wanted to continue "accelerating and intensifying" negotiations. "With no backstop there will be no agreement", he said.
That goes specifically for the sensitive issue of the EU-UK land border between Ireland and British-ruled Northern Ireland, where the EU wants a "backstop" solution in case of no deal, which includes terms that are anathema to London.
"Our responsibility is to be prepared for any and all the options", Mr Barnier told Germany's Deutschlandfunk radio on Thursday.
"On my side I am stubbornly optimistic that a deal is within our reach if both sides grasp the opportunity", Mr Raab said.