Reuters has cited two unnamed EU diplomats as saying that the UK's intention to override parts of its Brexit divorce agreement will have far-reaching consequences.
"'Pacta sunt servanda' meaning 'agreements must be kept' is a fundamental principle in international law. If the UK chose not to respect its international obligations, it would undermine its international standing", one diplomat pointed out.
They suggested that no one "would want to agree trade deals with a country that doesn't implement international treaties", something that the diplomat claimed "would be a desperate and ultimately self-defeating strategy".
Another EU source warned that without "correct implementation" of the Brexit-related withdrawal agreement, they "cannot imagine the EU would conclude a treaty with a country that does not abide by its treaty commitments".
Johnson Urges EU, UK to 'Move On' if Trade Deal isn’t Clinched
The remarks come as British Prime Minister Boris Johnson told reporters earlier on Monday that if no free trade deal between the EU and the UK is concluded by 15 October, both sides should "accept that and move on".
He expressed his country's readiness for any possible outcome from the Brexit negotiations with the EU.
"We're ready for any eventuality. We will get through this. It's absolutely vital that our partners understand that the UK is going to do what we need to do, if we have to have an […] Australia-style solution then that is what we will achieve and we will prosper mightily one way or the other", Johnson emphasised.
The UK prime minister added that London "cannot and will not compromise on the fundamentals of what it means to be an independent country to get it", vowing at the same time that Britain's "door will never be closed" and that the sides "will trade as friends and partners – but without a free trade agreement".
He spoke after UK Brexit negotiator David Frost said on Sunday that he is not scared of a no-deal outcome, and that the sides are completely prepared for the scenario. "If we can reach an agreement that regulates trade like Canada's, great. If we can't, it will be an Australian-like trading agreement and we are fully ready for that", he stressed.
The European Commission, for its part, signaled resolve to clinch a free trade deal with the UK as soon as possible, but added that such an agreement should ensure fair and open competition.
Fishing rights remain the main sticking point in the negotiations, which were launched in early March, with the EU seeking to retain access to British waters in a bid to fish there.
Earlier this year, the EU's chief negotiator insisted that a "new dynamism" is needed in the negotiations, while Frost argued that Brussels' proposed deal "contains novel and unbalanced proposals which would bind this country to EU law or standards".