SINN Féin MP Chris Hazzard has claimed the British government's UK Internal Market Bill is a "clear deviation from the Good Friday Agreement".
The former Stormont infrastructure minister made the assertion in a letter sent to the leaders of Westminster's main parties.
The South Down MP claimed not only would the legislation break international law in overriding aspects of the Withdrawal Agreement – as conceded earlier this month by Secretary of State Brandon Lewis – but it would also "drive a coach and horses" through Good Friday Agreement.
"The bill would give the British government the power to unilaterally impose its rules and regulations on the assembly, regardless of the views of the democratically elected MLAs and ministers in the executive," Mr Hazzard said.
"This is a clear deviation from the Good Friday Agreement and the political institutions it created."
He said there were clauses in the bill that would give the British government "powers to compel the executive and assembly to accept lower environmental, food safety and animal welfare standards".
Mr Hazzard said such a move would jeopardise the north's agrifood industry.
However, Ulster Unionist leader Steve Aiken has claimed interventions from the EU, the Republic and the US are "in danger of pulling the Belfast Agreement apart in the Brexit negotiations".
“The continuing commentary from Dublin, Brussels and Washington demonstrates either extreme naivety about the Belfast Agreement or else deliberate manipulation to sustain partisan pro-united Ireland positions which will do long term damage to relationships across these islands and particularly within Northern Ireland," he said.
"It's as if the Belfast Agreement only refers to one community within Northern Ireland and fundamentally ignores the imposition of a border down the Irish Sea – an internal border within a sovereign country."
He said unionists were "as entitled to have their rights protected as nationalists".
On Saturday, Sinn Féin leader Mary Lou McDonald said the coronavirus pandemic and Brexit had made the prospect of Irish unification part of "every day conversation".
Expressing her concern at how Ireland may emerge from the health crisis, she said work should start towards unification.
"Brexit and Covid-19 have made Irish unity part of every day conversation," she said in an online message.
"Growing our economy, protecting our health service, strengthening our agriculture require an all Ireland approach – so now is the time to progress this journey together."
Meanwhile, an opinion poll published yesterday indicated a slight increase in support for Sinn Féin and Fine Gael.
The Sunday Times-commissioned survey of more than 900 voters over 12 days to last Tuesday found support for Fine Gael up one point to 30 per cent, support for Sinn Féin up two points to 32 per cent and support for Fianna Fáil had fallen by one point to 19 per cent.
