Boris Johnson was adamant an extension will not be requested and the UK will leave the bloc at the end of the year regardless of the outbreak.
Face-to-face negotiations on the future relationship were scheduled to be held in London next week but were cancelled because of Covid-19.
The Prime Minister's official spokesman dismissed the possibility of an extension when asked at a Westminster briefing on Friday, adding: "In UK law, a request for an extension is not possible."
The Withdrawal Agreement that passed Parliament in December included a commitment that ministers will not request any further delay past the transition period that ends on December 31.
As the coronavirus outbreak worsened, the second round talks were cancelled in their planned form and officials were looking at alternative arrangements including videoconferencing.
Meanwhile, EU chief negotiator Michel Barnier said on Friday he had sent a draft treaty showing an "ambitious and comprehensive" future relationship is possible for member states.
This was leaked to various news outlets, days before the UK Government was expected to receive the finished version.
The draft said the UK must abide by Brussels' state subsidy rules and the European Court of Justice (ECJ) would hand down rulings to UK courts, according to those who had seen it.
It also reportedly included level playing field proposals on competition, tax, labour, social protection and the environment.
Officials in the UK privately made clear any document that required alignment or ECJ jurisdiction would not wash.
The PM has ruled out any kind of alignment with Brussels or accepting any ECJ jurisdiction.