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President of the European Commission Jean-Claude Juncker delivers a speech during a session of the European Parliament on Brexit on January 30, 2019 in Brussels, Belgium.

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U.K. and EU negotiators are unlikely to reach a trade agreement in the coming months, Jean-Claude Juncker, the former president of the European Commission, said at an event Tuesday.

The two negotiating teams started their eighth round of discussions over new trade arrangements on Tuesday a necessary step after the U.K. left the European Union in January and agreed to work toward a trade deal with the bloc, to be implemented in January 2021.

However, the trade talks have not made any significant progress so far and there are growing doubts that this will change in the coming weeks.

"The situation is not developing in the best direction possible," Jean-Claude Juncker, who led the executive arm of the EU between 2014 and 2019, said at an event hosted by financial services company Principal.

"No deal is the most possible and probable, the only outcome of the negotiations," Juncker, who often played a key role in prior Brexit negotiations, said.

Tensions between both sides of the English Channel have risen in recent days, with reports that the U.K. government is preparing to override parts of the Withdrawal Agreement the deal that allowed for its orderly departure and for a transition period throughout 2020.

Flouting parts of that agreement would diminish trust in trade talks; the EU has said that the implementation of the Withdrawal Agreement is a precondition to any trade deal.

"I trust the British government to implement the Withdrawal Agreement, an obligation under international law and prerequisite for any future partnership," Ursula von der Leyen, president of the European Commission, said Monday.

In addition, U.K. and EU negotiators have been unable to agree on state aid and new rules on fisheries since trade talks began earlier this year.

Speaking Sunday evening, U.K. Prime Minister Boris Johnson said that if they could not overcome these differences by October 15, "then I do not see that there will be a free trade agreement between us.

This would mean that as of January 1, EU-U.K. trade would follow the World Trade Organization rules. In practical terms, this could increase costs for companies on both sides.

According to Juncker, the U.K. government doesn't "understand that there is no way to be part of the internal market without agreeing to common rules."

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No-deal Brexit is the most likely outcome of trade talks, former EU chief says - CNBC