BELFAST — Boris Johnson is moving forward on his threat to override the Brexit withdrawal agreement with the European Union — a move that every living former British prime minister warns against — and the people of Northern Ireland again find themselves fearful that decisions made in London and Brussels could upend the hard-won peace and prosperity on their island.

Britain will leave the European Union at the end of 2020, with or without a new free-trade deal, Johnson promises. With just over three months to go before the end of a transition period, a pact between the sides seems as far away as ever. Relations between Europe and Britain have grown shouty, underlining the high stakes of the showdown, as Britain and Europe both struggle to recover from deep pandemic recessions.

Rachel Powell grew up in South Armagh, where during the Troubles a hard border between Northern Ireland and the Republic of Ireland was patrolled by British troops, often attacked by Irish republican militants. She said she’s deeply concerned about what will happen next.

“The British government has not got a clue about what it is like to live on the border, and it is again using it as a political football,” said Powell, a lobbyist with the Women’s Resource and Development Agency in Belfast.

The 1998 Good Friday Agreement brought peace to Ireland — and today the line marking the boundary between Northern Ireland and the Republic of Ireland is invisible.

Powell said border communities are “horrified” over the uncertainties and brinkmanship of Brexit.

In the past week, Johnson started to deploy martial language, asserting that the E.U. is plotting to “carve up our country” and to choke off food supplies with destabilizing new barriers between Great Britain and Northern Ireland.

“Unless we agree to the E.U.’s terms, the E.U. will use an extreme interpretation of the Northern Ireland protocol to impose a full-scale trade border down the Irish Sea,” Johnson wrote in the Telegraph newspaper.

Irish Foreign Affairs Secretary Simon Coveney dismissed Johnson’s claim that Europe wanted to “blockade” Northern Ireland as “totally bogus.”

Europeans are livid that Johnson has introduced a bill before Parliament that would breach key parts of the Brexit withdrawal treaty, an agreement Johnson signed only nine months ago, calling the pact “historic” and “fantastic.”

The deal seeks to finesse trade and customs issues to enable Northern Ireland to exit along with the rest of the United Kingdom but to preserve easy trade — and psychological ties — with the Republic of Ireland, which will remain part of the E.U. In the absence of a free-trade deal, there would probably have to be some kind of controls between north and south to levy tariffs and quotas.

Chief E.U. negotiator Michel Barnier said the Northern Ireland protocol signed last year represents no threat to the integrity of the United Kingdom. He tweeted, “We agreed this delicate compromise with [Boris Johnson] & his gov in order to protect peace & stability on island of Ireland. We could not have been clearer about the consequences of #Brexit.”

Martin O’Brien is a veteran peace-building activist from north Belfast, who worked to ensure that strong human rights were included in the Good Friday Agreement. “Brexit — and this government’s particular form of Brexit, a hard Brexit — was embarked on with no regard to the consequences for Northern Ireland,” he said.

O’Brien called the withdrawal agreement signed by the U.K. and E.U. last year “a carefully constructed mechanism to try to minimize the worst consequences” of Brexit.

“Now the government has decided to go back on the compromise and, again, that is hugely destabilizing,” he said.

Queen’s University Belfast professor Katy Hayward said, “We should be familiar with this by now, the stirring up of a political storm over Northern Ireland and its place after Brexit, but we are no less weary of it.”

She said that “the anxieties are only growing. . . . We face the prospect of the U.K. flagrantly breaking international law and using Northern Ireland’s position as justification for it. This sets a whole new precedent. And it bodes very badly for peace.”

During the Troubles, more than 3,500 people were killed, over half of them civilians.

Brandon Lewis, Johnson’s minister for Northern Ireland, admitted to the House of Commons last week that the new bill to amend the Brexit deal with Europe will “break international law,” but, he said, only in a “specific and limited way.”

Johnson’s move has set off alarms in the U.S. Congress, with House Speaker Nancy Pelosi (D-Calif.) warning that today’s open border between north and south must be preserved at all costs. Undermine the Good Friday pact, Pelosi said, and “there will be absolutely no chance of a U.S.-U.K. trade agreement passing the Congress.”

If Britain and Europe are unable to sign a trade deal by year’s end, then World Trade Organization rules would apply to goods traded between the E.U. and the U.K., with tariffs and quotas levied in both directions. Supply chains 40 years in the making could be derailed.

On the island of Ireland, there is growing anxiety that Johnson is hurtling toward a no-deal Brexit.

“Securing a ‘zero tariff, zero quota’ free-trade agreement with the E.U. remains critical to the future of U.K. businesses,” said Ann McGregor, chief executive of the Northern Ireland Chamber of Commerce and Industry. “We urge ministers to redouble their efforts to reach a comprehensive agreement with our largest trading partner at a crucial time in the negotiations.”

Former British prime ministers Tony Blair and John Major joined forces to condemn Johnson’s attempt to override the ­Brexit agreement, calling it “shameful.” They urged lawmakers in the Labour and Conservative parties to vote down the bill.

“It raises questions that go far beyond the impact on Ireland, the peace process and negotiations for a trade deal — crucial though they are. It questions the very integrity of our nation,” the pair wrote in the Sunday Times.

Since then, the bill cleared an initial vote in the House of Commons.

Sammy Wilson, a senior figure in the Democratic Unionist Party in Northern Ireland, said his group will table amendments to Johnson’s bill. But he dismissed as “utter bunkum” the former prime ministers’ worry that the Irish peace could unravel.

Wilson said Johnson’s bill will make it easier for Northern Ireland to do business with Great Britain with less paperwork.

Booth reported from London.