London's police chief has said he believes there is a link between the terrorists who bombed Madrid and al-Qaeda supporters in the UK capital.
Scotland Yard said it could not expand on Sir John's comments
Metropolitan Police commissioner Sir John Stevens told the Independent newspaper there was a "definite link" between extremists in both countries.
Scotland Yard confirmed it was looking into any connections but declined to give further details.
A series of bombs in rush hour Madrid last Thursday claimed 202 lives.
Friday's Independent reported that anti-terrorist officers and MI5 were trying to establish whether Islamic extremists based in the UK gave money to one of the prime suspects in the Madrid attacks.
Sir John said: "We believe there is a London link with what happened in Madrid."
Scotland Yard confirmed the comments reflected the current thinking in the capital's police force, but said specific details could not be discussed.
Earlier this week the commissioner said a terrorist attack on London at some stage was "inevitable".
Sir John said UK security services were working "three times harder than ever" in a bid to foil a possible attack.
On Friday, the home secretary is due to announce that an extra £15m would be given to police anti-terrorism units across the UK.