PM Boris Johnson says the UK’s actions “temporary and technical measures that we think are very sensible”.
The EU has begun legal action against the UK over its alleged breach of the NI Protocol.
It could lead to the UK having to defend its actions at the European Court of Justice.
The European Commission’s vice president said he hopes the issue can be resolved without further legal action.
Maroš Šefčovič said the EU’s preference is for “collaborative, pragmatic and constructive” political discussions.
The protocol is the part of the Brexit deal relating to Northern Ireland and has led to the creation of a new trade border between Northern Ireland and the rest of the UK.
Earlier this month, the UK government changed how the protocol is being implemented without EU agreement.
It delayed the introduction of new sea border checks on food, parcels and pets.
It also moved unilaterally to ease the trade in horticultural products across from Great Britain to Northern Ireland.
The European Commission has sent a letter of formal notice to the UK saying these actions breach the substantive provisions of the protocol as well as the good faith obligation under the Withdrawal Agreement.
It has asked the UK to respond within a month before it decides on further legal steps.
Mr Šefčovič has separately sent a “political letter” to David Frost, the UK minister in charge of Brexit.
It calls for the UK to enter into good faith consultations in the Joint Committee, the body which oversees the protocol, with the aim of reaching a mutually agreed solution by the end of this month.
Mr Šefčovič said: “The EU and the UK agreed the protocol together. We are also bound to implement it together.
“Unilateral decisions and international law violations by the UK defeat its very purpose and undermine trust between us. The UK must properly implement it if we are to achieve our objectives.”
Maroš Šefčovič is clear that the EU would prefer to resolve the Protocol diplomatically rather than legally.
But the Commission, sometimes styled as the guardian of the EU’s treaties, will feel that it has to put down a marker.
It pointedly says this is the second time in six months that the UK is set to breach international law in respect to a treaty it had made with the EU.
It will not want to create the impression to breach treaties is without consequence.
Speaking earlier, the prime minister said the actions taken by the UK were “temporary and technical measures that we think are very sensible”.
Asked on a visit to Coventry on Monday, Mr Johnson said: “The protocol is there to uphold and guarantee, to buttress the Good Friday Agreement and the peace process.”
It had always been “very important [that] the wishes and consent of both communities in Northern Ireland are properly reflected in the outcome and that it should guarantee trade and movement not just north-south but east-west as well”, he added.
The UK looked forward to discussions with its EU friends on the issue, said the prime minister.
The UK government had received the European Commission letters and would respond “in due course”, said a statement.
It described the UK’s actions as “temporary, operational steps” which were “lawful and part of a progressive and good faith implementation” of the Northern Ireland Protocol.
“Low key operational measures like these are well precedented and common in the early days of major international treaties,” it continued.
“In some areas, the EU also seems to need time to implement the detail of our agreements.”
Northern Ireland First Minister Arlene Foster said the legal action was “further proof that Brussels is closing its eyes to the serious problems the Protocol has caused for Northern Ireland”.
“Rather than showing concern for stability in Northern Ireland or respect for the principle of consent, Brussels is foolishly and selfishly focused on protecting its own bloc,” said the DUP leader.
“Not one single unionist party in the Northern Ireland Assembly supports this flawed Protocol, therefore Brussels’ claim to be protecting peace continues to ring hollow.
“Regardless of the reaction in Brussels, the Prime Minister must deliver the unfettered flow of goods from Great Britain to Northern Ireland.”
The Northern Ireland Protocol is part of the Brexit deal which prevents a hardening of the land border between Northern Ireland and the Republic of Ireland.
It does that by keeping Northern Ireland in the EU single market for goods.
That has created a new trade border with Northern Ireland and the rest of the UK.
Unionists oppose the protocol, arguing that it has damaged internal trade from GB to NI and poses a risk to the future of the UK union.
But anti-Brexit parties in NI say that it must be implemented in full, and that issues should be worked out through joint UK-EU processes.