European officials are pessimistic about a reset in post-Brexit relations with the UK, whoever becomes Britain’s next prime minister in September.

Whether it is Liz Truss or Rishi Sunak who are handed the keys to Downing Street on 5 September, officials in Brussels have little hope of a rapprochement with the new government.

More than six years after Britain voted to leave the European Union, relations have hit a post-Brexit low, as the UK government pushes ahead with plans for a unilateral rewriting of the Northern Ireland protocol, a linchpin of the post-Brexit agreement. The EU has said the plans – led by Truss, the foreign secretary – would breach international law and has threatened to tear up the post-Brexit trade deal.

One EU diplomat said there was nothing to suggest that Truss, the frontrunner, would abandon the approach she has pursued as foreign secretary. “If the UK government follows through with the plan already set out, I think it’s fair to say relations will get worse,” they said.

British and EU officials close to the long-stalled talks on the protocol, however, believe there is a window of opportunity for a new prime minister, once the contentious bill goes to the Lords, where it could be debated for months.