LONDON (Reuters) - Irish Prime Minister Micheal Martin warned Britain on Monday that a unilateral breach by London of post-Brexit trade rules for Northern Ireland, agreed with the European Union, would be "very serious".

       "Unilateral breach of the Protocol is very serious - an international deal ratified by British Parliament and approved by the PM," Martin said on Twitter.

       "It goes to the heart of the issue of trust."

       (Reporting by Andy Bruce; Editing by Kate Holton)