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British security agency are also monitoring all planes flying through the UK to destinations to the US and mainland Europe as part of the terrorist threat, media report said.
The warning of an impending Al-Qaeda attack was delivered in a letter sent to aviation chiefs, BBC and Daily Telegraph newspaper reported, adding that following this the security had been upgraded at all major UK airports and transport hubs as a precautionary measure.
The plot by the terror outfit believed to be based in Pakistan`s tribal belt may come in the form of car-bomb, suicide attacks or Mumbai-style carnage, the daily said, as it was designed to cause "large number of casualties".
The document warned that Al-Qaeda planned for "one or more attacks" against Europe, including the UK aimed at high-profile targets.
The terror alert has been sounded, BBC said, after increased "chatter" was noticed among terror suspects under surveillance of the intelligence agencies.
Quoting UK government security officials, the BBC said the overall national threat level was at "severe", which meant a terrorist attack is highly likely.
The British Home Office said if there was any intelligence of an imminent threat or a plot underway, the treat level would be raised to the highest level "critical".
The BBC said it had seen a copy of the letter sent from the Home Department to transport and aviation officers, explaining the increase in the threat level to the industry.
"The letter says there are indications Al-Qaeda may be considering an attack against a UK airport or aviation sector target," BBC said, describing the report as "credible".
Britain`s move comes after France and Germany have also beefed up security following intelligence warnings that terrorists were plotting a Mumbai-style attack on a European city.
European security and police agencies have been jittery in recent weeks after Iraqi officials claimed last month that captured militants had told them of terrorists plot of hitting targets in Europe in Christmas season.
BBC said officials were stressing that there was no threat of an imminent attack but precautionary measures were being taken.
"But what we can expect to see is a greater police presence, particularly at airports and large railway stations," BBC said quoting officials.
