Fears that Islamic extremists hacked the personal details of Croydon Council were unfounded, an investigation has concluded.

The Foreign Office and the council launched a joint probe in August after reports claimed the names, email addresses and passwords of council employees were among data leaked online by Isis.

Information relating to personnel at the Foreign Office, Stockport Council and the American military were also contained on the leaked spreadsheet, according to the Daily Telegraph.

The data was accompanied by a message addressed to western "crusaders", who were warned: "we are extracting confidential data and passing on your personal information to the soldiers of the khilafah, who soon with the permission of Allah will strike at your necks in your own lands!"

AUGUST: Investigation launched into claims Croydon Council staff data 'hacked by Isis'

But Croydon Council said in August it had found no evidence of a data breach, while the Croydon Guardian could find no reference to any of the authority's staff on the spreadsheet.

A council spokeswoman today confirmed its investigation had concluded the claims were incorrect.

She said: "Investigations via the Foreign and Commonwealth Office found no evidence that Croydon Council staff’s emails had been shared on websites, contrary to the reports of some national newspapers.

"Internal investigations also confirmed that there had not been a breach of our systems. We continue, however, to remain vigilant as ensuring all our data remains safe is a priority for us."

The data, posted online by a Twitter account claiming to represent Isis's hacking division, does appear to include the names, email addresses, passwords and phone numbers of thousands of US military staff, as well as some Foreign Office personnel and one woman who works for Stockport Council. 

But computer experts have raised doubts about whether the data released was obtained by hacking.

Security writer Troy Hunt said: "It appears hastily coupled together with inconsistent data structures and duplicate records."