The British government's Northern Ireland Secretary has said Brexit grace periods had to be extended due to levels of anger in the unionist community over the implementation of the deal that could undermine stability.

"If the unionist community feel that the Protocol is breaching the Good Friday agreement and moving away from the spirit of it, then we're in quite a dangerous place in terms of stability of not just the executive but the north-south institutions," Brandon Lewis told reporters.

Mr Lewis said the UK had to take unilateral action to extend grace periods otherwise shelves would have been empty in shops in Northern Ireland.

"If we hadn't, if we'd have had another set of empty shelves this week ... the fallout from that in the unionist community means the protocol would, I think, have been fatally flawed," he said.

The European Commission briefly threatened in January to impose emergency controls on vaccines crossing the land border, a step that Mr Lewis said had increased tension in the unionist community.

"We're still dealing with the fallout from that," he said.