THERESA May's visit to the border is "too little, too late", Michelle O'Neill has said.
Sinn Féin's deputy leader made the comments just hours before the prime minister is due to meet business owners.
"She is coming two years after the referendum, she is coming two years after negotiating with her own party," the Sinn Féin politician said.
"I am quite clear what she will hear today, she'll hear about the catastrophic implications of Brexit, the fear and trepidation of the business community in terms of what comes next for them.
"We can't withstand being outside the customs union and the single market.
"Theresa May needs to realise that we will not be collateral damage her for own reckless Tory agenda."
Ms O'Neill also criticised Mrs May's Conservative Party over Brexit and said she is concerned that the prospect of the UK leaving the EU without a deal seems likely at this stage.
"It's been a Tory party trying to pander to two sides of an argument, it's been a Tory party embroiled in confusion, chaos and contradiction. I have no doubt that will continue, I don't think one hand knows what the other is doing," she said.
"I think it is increasingly likely (that there will not be a deal), I think that's very worrying, so in the event of that crash, in the event of a hard border, the only security we have is the backstop, that's why we need it written down in legal certainty."
She added that if the British government tries to renege on its commitment to the Irish protocol, it is "up to the EU and the Irish Government to hold them to account for the commitment they made".
Ms O'Neill, along with Sinn Fein President Mary-Lou McDonald and Newry and Armagh MLA Conor Murphy, is set to meet Mrs May tomorrow.
