SINN Féin and the SDLP have called on Ian Paisley to resign after he faces suspension from the House of Commons for failing to register two family holidays paid for by the Sri Lankan government.
The North Antrim MP has already apologised for what he said was his "unintentional failure" to register the holidays, which he estimated were worth £50,000.
Mr Paisley could now face a by-election if 10% of the electorate in his constituency sign a recall petition. In last year's General Election Mr Paisley polled more than 28,000 votes, nearly 20,000 more than his nearest rival.
The Parliamentary Commissioner for Standards has recommended that Mr Paisley be suspended from the Commons for 30 days.
The commissioner said the cost of the holidays may have been "significantly more" than Mr Paisley's estimate.
The holidays in 2013 included business-class air travel, accommodation at first-class hotels, helicopter trips and visits to tourist attractions for the MP and his wider family.
Political reaction
Sinn Féin's Foyle MP Elisha McCallion said: "This is an issue of integrity in government and in public office.
"Ian Paisley Jnr should step down and resign with immediate effect. If he is not prepared to do the right thing then the DUP must demand his resignation."
She added: "If Ian Paisley or the DUP do not do the right thing then it is entirely legitimate for the electorate of North Antrim to exercise its right to force a by-election."
The SDLP said Mr Paisley's position was "untenable". MLA Colin McGrath said a 30 day suspension was not enough.
"The SDLP will be seeking a recall petition to dismiss Paisley from Westminster, and should this trigger a by-election in North Antrim, Mr Paisley should not challenge the seat again," he said.
'Serious and shameful' breaches of code of conduct
TUV leader Jim Allister called on Mr Paisley to refund the Sri Lankan government the money it spent on hosting him, accusing the MP of "serious and shameful" breaches of Parliament's code of conduct.
"It is good that he has been held to account, but in bringing this shame on himself he has not only let himself down but left North Antrim with no voice and vote in Parliament at this critical time for Brexit," said the North Antrim MLA.
"If any key vote on Brexit is lost because of Ian Paisley's antics, then that will be his shameful legacy for all time."
He added: "It now falls to the DUP, having said it would await the outcome of this investigation, to demonstrate where it stands on such disreputable actions by one of its MPs."
A DUP spokesman said: "The party has noted the House of Commons Standards Committee's report on Ian Paisley MP. These matters will now be considered by the party officers."
Alliance Assembly member Paula Bradshaw also urged Mr Paisley to consider his position.
"Mr Paisley has let both himself and his constituents down, calling into question his personal integrity and the ability of voters to trust him," she said.
Ulster Unionist Party leader and North Antrim MLA Robin Swann said the episode was "embarrassing" for the constituency.
"The Standards Commissioner has made her recommendations and I wait to see what action is taken by the DUP in light of this," he said.
Read the full report here
General Election 2017
The DUP won 58.9% of the vote in North Antrim in last year's General Election.
Ian Paisley polled more than 28,000 votes, 20,000 more than his nearest rival, Sinn Féin's Cara McShane, and about 10,000 more than his total last time round.
How the 2017 elections results compare to 2015:
