Chelsea FC have said they are aware of incidents involving their fans amid reports that two supporters have been stabbed in Istanbul.
The Press Association had earlier reported two supporters had been attacked on Tuesday evening but neither needed hospital treatment.
The west London club tweeted a warning to its fans not to use the Turkish city's Metro as it was "not safe".
Chelsea play Istanbul side Galatasaray in the Champions League on Wednesday.
Chelsea had tweeted: "The club is aware of 2 incidents involving fans in Istanbul last night and is working with relevant agencies to best ensure safety of all"
The British Embassy in Ankara confirmed it was aware of the reports and was discussing them with the club and Turkish authorities.
The Foreign Office confirmed it was investigating claims of trouble around Taksim Square, in the heart of the city's commercial district, but that it had "no confirmation of an incident".
English fans have been victims of violence in Istanbul before, with Leeds fans Christopher Loftus and Kevin Speight stabbed to death before a UEFA Cup tie in 2000.