Sinn Fein would block the formation of a new Stormont Assembly if the final Brexit deal were to hand a veto to any party, Mary Lou McDonald has indicated.

The Sinn Fein president warned the UK Government not to try to renegotiate the agreement in response to DUP demands for changes to the way the Assembly consents to post-Brexit arrangements on the island of Ireland.

The DUP insists that consent must be on the basis of a majority of unionists and a majority of nationalists agreeing, rather than a straight majority vote.

Boris Johnson gave ground on the consent issue in last week’s negotiations with the EU, having originally proposed a mechanism in line with the DUP’s thinking.

Assembly consent for extending an initial four-year period for the arrangements, which include the creation of an all-island regulatory zone on goods, will now be done on the basis of a simple Stormont majority.

Ms McDonald cautioned the Government not to consider another change of tack in an effort to win DUP support for the deal.

“We need clarity on Brexit,” she said in Belfast on Monday.

“So let me say this very clearly, from our perspective, if the British Government were to make the fatal error of granting a veto to unionism over Brexit protections, if they were foolish enough to insist that that veto could be exercised through the Assembly, then there will be no Assembly here in Belfast.

“The days of vetoes to deny individual rights, to deny human and civil rights, the days of using vetoes to deny rights and protections to the people on this island, north and south, are over.”

According to the new Brexit deal, the arrangements will come into effect at the start of 2021 and, after an initial four-year period, Stormont Assembly members will vote whether to continue to apply them.

That vote will be conducted on a simple majority head count and will not require the support of a majority of unionists and a majority of nationalists under the contentious “petition of concern” mechanism.

This means the DUP would not have the chance to exercise a veto.

If the vote is carried, the arrangements will be extended for another four years.

If it transpires that a majority of unionists and a majority of nationalists do ultimately vote in favour of the move, then the extension period will be for eight years.

If members vote to come out of the EU arrangements, there would be a two-year cooling-off period before that happened.