Abdelhamid Abaaoud's alleged trip to Britain could add to pressure on the interior ministry, which is struggling to tighten border controls amid high-profile reports of Britons going to Syria to join the Islamic State group.
The newspaper said photos of British landmarks were found on Abaaoud's phone after he was killed in a raid in Paris just days after the November 13th coordinated gun and bomb attacks in the French capital that killed 130 people.
The Guardian cited no sources and the interior ministry declined to comment.
Abaaoud entered Britain through a ferry port in England's southeast, the newspaper said, giving no dates.
The Guardian said he had met several jihadists in London and in Birmingham in central England, but that it was not clear whether the photos on his phone were part of plans for an attack.
He had been the subject of an arrest warrant issued by Belgium, where in July he was sentenced in absentia to 20 years in prison for recruiting jihadists to fight in Syria.
The 28-year-old of Moroccan origin had in the past boasted of evading police dragnets in Europe, and taunted European authorities from what was assumed to be an Isis base in Syria.
He also bragged about escaping from Europe after Belgian police shot dead two of his fellow jihadists in the eastern town of Verviers as they broke up a cell planning attacks on security personnel last year.
Abaaoud had first popped up on the radar of Belgian security forces after featuring in an Isis video released in 2014, laughing as he drove a car which dragged mutilated bodies behind it.