Irish nationalists were "sold a pup" when they agreed to the Good Friday Agreement, according to a Sinn Fein MP.
Francie Molloy posted on Twitter that there has been no commitment from Dublin or Westminster to deliver for nationalists or republicans, describing the deal as "just a bluff".
Mr Molloy told this newspaper the comments were written "out of frustration" after Taoiseach Micheal Martin dismissed the idea of a border poll.
Speaking during his first trip to Northern Ireland since taking up the post, Mr Martin last week said he does not believe a border poll is a possibility within the lifetime of the current government.
"But what I would like to see is significant work under way in terms of what a shared island would look like," he said.
"Rhetoric is easy, statements are easy, we can all speak to our base.
"But there's a hell of a lot of work and a lot of practical stuff can get done."
However, Mr Molloy has hit out at the position and said he believes the time is right for a border poll.
The Mid Ulster MP said he believes any reticence about holding a referendum on the issue is down to a fear that a majority of people will vote in favour of a united Ireland.
"I do think that after Brexit, the governments do actually realise that it could happen," he said.
"My comments are down to the fact that the Irish Government are signatories to the Good Friday Agreement, yet they aren't doing anything to implement the border poll.
"Both Dublin and Westminster signed up to the Good Friday Agreement, they're both guarantors, yet Micheal Martin is dismissing the idea of asking for a referendum for Irish unity.
"I think that it's more than the right time for a poll because of the referendum on Brexit. On that occasion, 11 out of 18 constituencies voted to remain in the European Union and the only way that can happen now is through an all-Ireland structure.
"I am one of the people who was involved in the Good Friday Agreement and I'm totally committed to it but at the moment, there is a failure by both governments to ensure that it has been fully implemented.
"There is a sense of frustration from a republican and nationalist point of view that they put faith put in the Good Friday Agreement 20 years ago and it still hasn't been delivered.
"It's very clear that Northern Ireland is being left behind by Fianna Fail who claim to be a republican party."
However, the DUP's Gordon Lyons has rejected the suggestion that it is the correct time for a referendum on a united Ireland.
The East Antrim MLA said a border boll would be "divisive and would take Northern Ireland backwards".
He continued: "Our focus needs to be on moving forward, getting out of the pandemic and protecting lives and livelihoods rather than spending time chasing a border poll that would be detrimental to stability and recovery.
"Being part of the UK is hugely beneficial to everyone in Northern Ireland and that has been especially apparent during this pandemic.
"Whether it was our NHS saving lives or the job retention scheme or other economic measures saving livelihoods, being part of the UK was critical for Northern Ireland."
During his visit to Belfast, Mr Martin met the First and Deputy First Ministers, Arlene Foster and Michelle O'Neill, and Northern Ireland Secretary Brandon Lewis.
January's 'New Decade, New Approach' deal pledged to "turbo-charge" connections between Dublin and Belfast and Mr Martin said he would work towards getting the "over the line".