The UK government has not been "even handed" when it comes to resolving the row over the Northern Ireland Protocol, Leo Varadkar has said.
The tánaiste (Irish deputy PM) accused Number 10 of "siding" with unionists in seeking to scrap parts of the deal agreed in 2019.
The DUP has said the protocol damages Northern Ireland's place in the union.
But Mr Varadkar said the UK government's bid to unilaterally change the protocol was a "strategic mistake".
His comments come just days after MPs gave their initial approval to the government's Northern Ireland Protocol Bill.
The proposed piece of legislation would allow ministers to change the part of the UK-EU 2019 deal - known as the Northern Ireland Protocol - that introduced post-Brexit checks on goods sent from Great Britain to Northern Ireland.
Those were designed to avoid checks at the UK's border with the Republic of Ireland, but are unpopular among unionists in Northern Ireland.
This week, Foreign Secretary Liz Truss said the protocol had "many practical problems", and had created "a growing sense that the rights and aspirations of some parts of the community are being undermined".
In a wide-ranging interview with BBC Northern Ireland's The View programme, Mr Varadkar said the EU would "not be threatened" by the UK's approach to the ongoing stand-off.
"The British government had given commitments in the past that it would be even handed," he said.
"I don't think that's the case when it comes to this government.
"That's a strategic mistake for people who want to maintain the union - to continue to impose things that a clear majority of people don't want means more people will turn away from the union.
"That's a peculiar policy coming from a government that purports to want to defend the union."
He also cited a letter written by assembly members (MLAs) at Stormont to the British government urging it not to proceed with the controversial protocol bill, saying Number 10 was ignoring the "democratic" views of a majority of parties.
Mr Varadkar was previously Ireland's taoiseach (prime minister) when the initial Brexit negotiations took place.
He is due to resume the role at the end of this year, under an arrangement that created the current coalition government in Dublin.
The tánaiste said that in his political lifetime, he had "never seen relations as bad" with UK ministers.
"We have a British government that doesn't want to work hand in glove with the Irish government, it's not even handed, it's a government that wants to continue to have rows with the EU even though they've left," he added.
Mr Varadkar rejected suggestions from Ms Truss that the EU had left the UK with no option but to move forward with the protocol bill.
"I think trust needs to be restored, the best way they can do that is by de-escalating this.
"Even if you have difficulties trusting someone, you still have to try to come to an agreement. If we can't with this government, then a future government."
The full interview with Leo Varadkar will be broadcast on The View tonight at 22:40 BST on BBC One Northern Ireland.
What is the Northern Ireland Protocol?
- The Northern Ireland Protocol is part of the Brexit deal: it means lorries don't face checkpoints when they go from Northern Ireland (in the UK) to the Republic of Ireland (in the EU)
- Instead, when goods arrive in Northern Ireland from the rest of the UK (England, Scotland and Wales), they are checked against EU rules
- The UK and the EU chose this arrangement because the Irish border is a sensitive issue due to Northern Ireland's troubled political history
Read more: A simple guide to the Brexit border problem