The Brexit Secretary said negotiations on the EU and UK’s post-Brexit relationship may “creep beyond” the meeting due to take place on 18 and 19 October.
The admission comes just a day after Theresa May’s spokesman told journalists the Government was “working to the October deadline”.
Speaking in front of the Lords European Union Select Committee, Mr Raab said a deal between both sides could still be reached if Brussels “matched” the UK’s efforts.
“It’s important as we enter the final phase of the negotiations to the lead up to the October council and the possibility that it may creep beyond that, we want to see some renewed energy,” he said.
He added: “I’m confident that a deal is in our sights. We are bringing ambition, pragmatism and energy and if it is matched, as I expect it will be, then we get a deal.”
Pressed on the length of delay he thought was possible, Mr Raab told the peers: “My starting point is March next year when we are leaving the EU and I work back from there.
“We are aiming for the October council but there is some measure of leeway.”
The near-two hour grilling coincided with what appeared to be a dialling down of rhetoric from EU chief Brexit negotiatior Michel Barnier.
He told an audience in Berlin that the EU was “prepared to offer a partnership with Britain such as has never been with any other third country”.
“In addition to the agreement, we must publish in October or November a very important political declaration on the future relationship, that we want with the UK to be as close as possible,” he said.
Labour MP Gareth Thomas, speaking on behalf of the anti-Brexit People’s Vote campaign, said: “It doesn’t matter who’s driving the Brexit clown-car, the wheels are still falling off.
“Dominic Raab has taken over from David Davis, but his admission today that the original October deadline for a deal may be missed means we’re fast running out of time to avoid total chaos.”
Mr Raab and Mr Bariner are due to meet in Brussels on Friday to cap off the latest round of negotiations between the two sides.
Elsewhere at the Lords committee, Mr Raab batted away claims reported in the Guardian that he was frustrated over the lack of face-to-face talks with an apparently difficult-to-pin-down Mr Barnier.
He said: “In relation to whatever tittle tattle may appear in whatever newspaper, I shall be over in Brussels tomorrow evening for a long, substantive meeting on Friday.”
However, he risked angering EU officials by claiming Britain could withhold some of the agreed £39bn payments to the EU if a deal is not reached before the UK leaves in March next year.
“I don’t think it could be safely assumed on anyone’s side that the financial settlement that has been agreed as part of the withdrawal bill would then just be paid, in precisely the same shape or speed or rate, if there was no deal,” he said.