Britain’s Prime Minister Andy Burnham exchanged messages with someone impersonating the White House chief of staff Susie Wiles, who is one of US President Donald Trump’s closest advisers, a source with knowledge told CNN.

It’s unclear when the exchange happened or what was discussed but Politico, which first reported the breach on Monday, said the British embassy in Washington was so concerned it raised the incident with the White House. Downing Street told CNN it does not comment on national security matters.

CNN understands that the communication was in the form of messages and “no messages of significance” were exchanged; Burnham quickly became suspicious, and the incident was reported to the proper authorities.