One of the perpetrators of the deadly Paris attacks visited London and Birmingham earlier this year to meet people suspected of plotting terror activity in Britain, The Guardian reported Saturday.
The newspaper, citing counter-terrorism officials, said the unnamed attacker had managed to enter Britain and travel to the nation's two biggest cities, before heading back to continental Europe undetected despite heightened security.
"In both cities he met with people suspected of having the intention and capability of plotting or assisting terrorist activity against the UK," the paper reported, describing him as an "ISIS (Islamic State) militant".
The suspects that he visited are under investigation by Britain's domestic intelligence agency MI5 and police counter-terrorism officials, it added.
The paper said that "months later" the man was part of the November 13 Paris attacks that killed 130 people.
Spokesmen for both the Home Office and the Metropolitan Police told AFP on Saturday that they could "neither confirm nor deny" the story.
The Met has been working closely with French colleagues since the devastating attacks.
Britain's "severe" threat level has meanwhile been in place since August 2014 and was unchanged after the Paris killings. The alert level means an attack is "highly likely", while the next level, "critical", means an attack is imminent.
Separately, the Wall Street Journal said Friday that the alleged ringleader of the Paris attacks, Abdelhamid Abaaoud, had links with several people in Birmingham in central England's West Midlands.
Abaaoud, a Belgian of Moroccan descent, was killed in a police raid five days after the November 13 attacks, along with a cousin and an unidentified third person, according to French authorities.
The WSJ also reported that at least one person connected to the attacks was thought to have travelled to Britain beforehand.
Officials told the US paper that some of the Birmingham-based people were also of Moroccan heritage.
In reaction to the reports, West Midlands Police said Saturday that it was aware of the speculation "regarding the Paris terror attackers and potential contact with people or places in Birmingham".
West Midlands Police assistant chief constable Marcus Beale added: "The West Midlands counter terrorism unit is working hand-in-hand with counter terrorism colleagues in London, the national CT network and security services to provide support to the French and Belgian investigations and of course to address any associated terrorism threat to the UK.
"We work tirelessly to counter terrorism. Our absolute priority is to ensure the safety and security of the people who live, work and visit the West Midlands area."
Islamist extremists have surfaced before in Birmingham, Britain's second-largest city. For example, Junaid Hussain, who has been identified as a high-ranking Islamic State group operative, was from Birmingham.
Belgium said Friday it was searching for two new "armed and dangerous" men who used false ID papers to help fugitive Paris attacks suspect Salah Abdeslam travel to Hungary in September.