Tense statements were released by both EU and UK leaders in the run-up to next week's negotiations on the Northern Ireland protocol and the next phase of Brexit discussions.
The European Union's ambassador to the United Kingdom said on Sunday that levels of trust between the two powers was low, but added that he was confident it would improve.
"The levels of trust are low right now," Joao Vale de Almeida told the Times Radio.
"We ultimately need to re-establish a minimum level of trust that allows us to find solutions, but this being said...I remain confident that we can get there."
Meanwhile, David Frost, the UK's Brexit minister, wrote that time was "starting to run out" in the review of the operation of the Northern Ireland protocol.
"In 2019 we agreed, as a huge compromise and for the greater good, to control certain goods movements within our own country and customs territory," Frost wrote in an opinion piece published in the Financial Times on Sunday afternoon.
"If that situation is not to be totally unsustainable we need to be able to do so in ways which do not disrupt everyday life and which respect everyone’s identity and interests.
"We continue to work for negotiated solutions which achieve this. But time is starting to run out. We need to see progress soon. I hope we can this week."
The Northern Ireland protocol was agreed in 2019 in order to sidestep the "backstop" and give a clear path for the UK's exit from the EU.
In order to do that, the pair also wanted to avoid a hard border on the island of Ireland and protect the Good Friday Agreement. Northern Ireland is still privy to some of the rules of the EU single market, including checks on goods entering from Great Britain. Brussels say these are necessary.
Frost has previously argued for a lighter touch — saying that checks had led to "delays and complexity" for businesses, and concerns among unionists.
Officials have also previously said that considerable compromise will be needed in order for the protocol to work.
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