Pro-Remain parties urging EU to stay firm on Ireland border 'backstop'
Sinn Fein could influence how Brexit unfolds in Parliament if it took its seats in the House of Commons and stop engaging in "stunt politics", the DUP has claimed.
Nigel Dodds was speaking after delegations from Sinn Fein, the SDLP, Alliance and the Greens travelled to London to meet with Labour leader Jeremy Corbyn and other opposition parties, including the Liberal Democrats, the SNP and Plaid Cymru.
Prime Minister Theresa May refused to meet them.
After the meetings, Sinn Fein described the trip as a "crucial show of unity" and underlined how "the majority of people" in the north opposed Brexit.
Michelle O'Neill emphasised how the backstop - which would see Northern Ireland remain part of the EU's customs union and single market - had to be permanent in any withdrawal agreement.
She also said that in the event of a no-deal Brexit in March there had to be a vote on a united Ireland, and her party would be pushing for one.
However, the DUP described it as a "classic example of stunt politics", saying Sinn Fein would have more impact if its MPs took their seats.
The Liberal Democrats also raised Sinn Fein's long-standing abstentionist policy as hampering the pro-Remain arguments in parliament during its meeting with the party.
DUP deputy leader Mr Dodds pointed to how many crucial Brexit votes had passed on the slimmest of margins. One vote passed in the Brexiteers' favour by just three.
He said: "At a time when crucial decisions are being made for Northern Ireland and the whole of the UK, Sinn Fein continue to boycott not just Westminster but the Assembly and Executive also.
"If they really cared or believed their own rhetoric they would end the boycotts but they are more interested in political stunts.
"On Brexit our voice and votes are counting whereas the boycotters render themselves increasingly isolated.
"The efforts of certain parties today to use Brexit to undermine the Union must not succeed. Their message is one which actually undermines the principles of successive agreements they purport to subscribe to."
On Sinn Fein not taking its seats, Liberal Democrat leader Vince Cable added: "We can't tell them what to do and they have an historic mandate which we understood. Obviously it would be desirable for Northern Ireland to be fully represented in the House of Commons and we raised this issue in an amicable and business-like way."
When it was raised with Sinn Fein outside Parliament, Mrs O'Neill pointed towards the internal civil war within the Conservative Party between its Remain and Leave wings.
"We can analyse many hypothetical situations," Mrs O'Neill added.
"What's important in the here and now at this crucial point is we make our voices heard."
She also warned that a hard border, with infrastructure at the frontier, could become a target for dissidents.
"While I am very cautious not to over-egg that, I think it is important that people understand that's exactly what potentially could happen," she said.