By Ece Toksabay and Nevzat Devranoglu

ANKARA (Reuters) - The number of foreigners visiting Turkey plummeted more than 40 percent in June, official data showed on Thursday, marking the biggest drop in at least 22 years as tensions with Russia and a series of deadly bombings kept tourists away.

The decline is the latest in a string of bad news for the economy as political and security concerns erode foreign investor confidence.

An attempted military coup this month has further bruised the outlook for Turkey's economy, once seen as a stand-out among emerging markets.

Some economists have forecast that tourism revenue - a pillar of the economy - could drop by a quarter this year, costing around $8 billion, or the equivalent of 1 percent of gross domestic product.

The drop-off is also likely to continue.

In late June, 45 people were killed in bomb and gun attacks at Istanbul's main airport, an attack authorities have blamed on Islamic State militants. This month, a faction of the military attempted to overthrow the government in an abortive putsch that left at least 246 people dead excluding the plotters.

On Thursday, British travel company Thomas Cook <TCG.L> cut its full-year profit target, after the failed coup forced its customers to change holiday plans. Turkey's Aegean and Mediterranean resorts and beaches are traditionally popular with European holidaymakers.

Tourist arrivals fell 40.86 percent year-on-year in June, with 2.44 million people arriving during the month, data from the Tourism Ministry showed. It was the biggest drop on record, according to the data, which goes back to 1994.

The previous record was set in May, when tourist arrivals fell 34.7 percent. Tourism has been hit hard by tensions with Moscow after Turkey shot down a Russian jet over Syria last year.

While Ankara and Moscow have recently started to rebuild ties, tourist arrivals from Russia dropped 87 percent in the first six months of the year, the data showed.

(Reporting by Ece Toksabay and Nevzat Devranoglu; Writing by David Dolan; Editing by Gareth Jones and Michael Georgy)