John Bolton, U.S. President Donald Trump's national security adviser, arrived in London on Sunday for talks at which he is expected to urge Britain to toughen its stance on Iran and Chinese telecommunications firm Huawei.
The impasse leaves Britain facing an exit without any formal transition period or legal agreement covering issues such as trade, data and border policy.
"Brexit should not impact the strength of our European partnerships", said one senior British government source familiar with the activities of one spy agency, adding that most intelligence-sharing was anyway conducted outside EU institutions.
Trump looks forward to meeting with Johnson "in the near future", the White House said in a brief statement.
Bolton told reporters afterward that a piecemeal approach to trade negotiations is "not unprecedented" and talks between the USA and the United Kingdom could concentrate first on areas where the two sides agree. -British "special relationship", during its monumental geopolitical shift.
As of now, Bolton will not meet with Johnson - who was named the UK's new leader on July 23 once May was forced into resigning after losing the support of her Cabinet, many of whom were fed up with her inability to secure the UK's departure from the European Union. They are ready to go.
The central message Bolton was delivering is that the United States would help cushion Britain's exit from the European Union with a free trade deal that is being negotiated by U.S. Trade Representative Robert Lighthizer and his British counterpart, Liz Truss.
He argued that there would be enthusiastic bipartisan support in Congress for speedy ratification at each stage.
"That would then lock that in and when the other areas that might be more hard were concluded later, you could combine it in one overall agreement. This government does. We're very happy about it".
A senior government source said Downing Street believes the first legislative showdown over no deal will be on 9 September, when parliament is due to debate a progress report on power-sharing in Northern Ireland.
Mr Bolton also said issues like security in Iran, and fears over Huawei's involvement in the UK's 5G network could wait until after Brexit to be resolved.