Washington lawmakers on Thursday expressed deep reservations about Dubai Ports World’s £3.9bn acquisition of P&O, the UK-based port operator, on the grounds that the deal represented a potential national security threat, and demanded that the White House re-open a regulatory review of the deal.

In a letter to Treasury secretary John Snow, Senator Richard Shelby, an influential Alabama Republican, stopped short of calling for the deal to be blocked, but said the transaction merited further scrutiny, potentially raising complications for DP World’s bid. Mr Shelby is expected to call for a hearing to discuss the issue in coming weeks.