Islamists that claim to support Isis have reportedly used Twitter to threaten an attack transatlantic flights from Heathrow.

An alert from counterterrorism organisation SITE Intelligence Group warned that a “pro-Islamic State” account had issued threats against aircraft “flying from Heathrow to the US” to coincide with the July 4 American Independence Day holiday.

The alert warned of a potentially deadly device being placed at Heathrow, Los Angeles airport or JFK in New York, The Times reported.

Transport minister Lord Ahmad of Wimbledon urged the public to be cautious.

He said: “We all need to be vigilant to the threat of terrorism. In the UK we keep all aspects of aviation security under constant review and work closely with our international partners to mitigate risks.

“We will continue to take all steps necessary to keep the public safe but for security reasons we do not comment in detail on specific measures or operational matters.”