The Ulster Unionist Party's decision not to agree an electoral pact in the upcoming poll is "bonkers", the DUP's chief whip has said.
Sir Jeffrey Donaldson said UUP leader-elect Steve Aiken's move would mean a "unionist dogfight" that could put at risk unionist seats to Sinn Féin.
Political parties are now preparing for a general election campaign.
On Tuesday MPs voted for a 12 December poll, which will be the third election in Northern Ireland this year.
The 10 Democratic Unionist Party (DUP) MPs voted in favour, while Independent MP Lady Sylvia Hermon voted against.
The legislation will later begin its passage through the House of Lords, where it is not expected to be opposed.
Unionist parties have traditionally agreed electoral pacts in certain constituencies in order to maximise the number of unionist MPs at Westminster.
They feel that if the unionist vote is split between them it makes it more likely a nationalist candidate will take the seat.
Mr Aiken has said his party will stand in all 18 constituencies this time, a move which sparked concern from others within unionist circles that key seats could be at risk.
DUP leader Arlene Foster said if there was not a pact, North Belfast MP Nigel Dodds could lose his seat and accused Mr Aiken of wanting to "hand away seats to Sinn Féin".
In Fermanagh and South Tyrone a unionist pact secured the UUP a seat for Tom Elliott in 2015 - but he lost it to Sinn Féin's Michelle Gildernew two years later.
However, Mr Aiken said unionist voters needed to be offered a choice and that his party could not be seen to criticise the DUP but then agree an electoral pact with them.
Speaking on BBC's Good Morning Ulster programme on Wednesday, DUP chief whip Sir Jeffrey Donaldson urged Mr Aiken to change his mind.
He said: "I just don't understand where Steve Aiken is coming from, that he proposes we have a unionist dogfight in the middle of the most important election in decades.
"I don't think they stand a chance of winning the seat in the constituencies where they will split the vote."
Meanwhile, Sinn Féin Vice-President Michelle O'Neill told the same programme that her party was not ruling out the possibility of pacts with other pro-remain parties in Northern Ireland.
She said a single pro-remain candidate would be best placed to win several seats in the Belfast constituencies.
However, she added that despite conversations with the SDLP and Alliance leaders, there had been no indication of that happening.
Speaking on the Stephen Nolan show, the SDLP's Claire Hanna said that the upcoming election is "about representing the remain voice in Northern Ireland and challenging the most extreme forms of Brexit."
The Alliance Party's deputy leader Stephen Farry also told the programme that his party would not be sacrificing any of their candidates to boost the chance of letting a remain candidate win in certain constituencies.
He said: "We don't think of voters as a commodity that political parties can trade. The electorate in Northern Ireland is sophisticated and they can work out what the best forward is themselves."
Who will run in the general election?
There are 18 Westminster constituencies in Northern Ireland, meaning 18 seats out of 650 in the Commons are up for grabs.
The DUP holds 10 seats, Sinn Féin has seven - but its MPs do not take their seats due to a long-standing policy of abstentionism - and independent unionist MP Lady Hermon holds North Down.
It is not clear if Lady Hermon will run again to retain her seat. She faced stiff competition for it from the DUP's Alex Easton in 2017.
Another question some are already asking is whether Alliance leader Naomi Long will stand in East Belfast, to try and reclaim the seat from DUP MP Gavin Robinson.
She held it between 2010 and 2015, when she lost it to Mr Robinson. He successfully retained it in 2017.
Mrs Long is now an MEP in Brussels, having won the seat for Alliance for the first time in May - but given her party's surge in that election and the council elections in May, she may be tempted to try to secure Westminster representation for Alliance again.
Meanwhile, SDLP leader Colum Eastwood has confirmed he will run in the party's traditional stronghold of Foyle, in a bid to reclaim the seat from Sinn Féin's Elisha McCallion, who won it for the first time in 2017.