Could it be an undeclared chemical spill by a secretive branch of the military? Or perhaps even the stench from bodies left over from the Black Death? Or is it just … burgers?

Deep below London it is a mystery that been getting up the nostrils of tube users already struggling in recent weeks from rising temperatures.

Questions about what lies behind a smell – variously described by commuters as “vile”, “revolting” and reminiscent of vomit – at Kennington station has prompted an investigation by transport authorities.

Among those querying the stink was the comedian, Rachel Parris, who registered her disgust in a tweet to Transport for London (TfL): “Londoners; sorry to be gross but does anyone know why Kennington station now permanently smells of sick? Like, for months now? V strong?”

TfL responded: “It’s a good question and one which quite frankly we don’t know the answer to. We are still investigating and hope to have identified the source and a solution soon.”

Parris has not been the only traveller to note the stench emanating from the 129-year-old station, which lies on the Tube network’s notoriously stuffy Northern line.

One commuter reported that the smell could be detected by passengers on carriages passing through the south London stop.

Another regular user of the station echoed that complaint last week, describing the smell as “strange”, and “sickly sweet” and saying: “I think anyone going down there will know what I mean – it really stings the nostrils. Is it dangerous?”

Responding on its Twitter account last week, TfL promised that the drains were due to be cleaned later in the evening on 8 August and that the problem should be resolved. But Parris’s exchange with the transport operator suggested that those efforts failed, as did another complaint on Sunday: “It’s getting worse and worse.”

The criticism has come after temperatures reached as high as 42C on sections of the London underground, which is mostly without air conditioning. The hottest line when measurements were taken on one occasion towards the end of last month was the Bakerloo. It was followed by the Central line on 40C and the Jubilee and Victoria lines (38C). The Northern line reached 37C.

While passengers appear to agree that something smells bad, there is no consensus as to exactly what the source is. One commuter coping with the smell claimed that a particular section of the station had long featured a powerful odour, arguing: “The northbound Charing Cross branch platform at Kennington smells of Burger King. It has always smelled of Burger King.”

The fast-food chain has been blamed for unexplained odours on the underground before. In 2017, claims that one of its restaurants lay behind the distinctive smell of one District line platform began gaining traction on social media as users pondered the question: Why has Victoria tube station started to smell like roast meat?

One answer was that the District line – unlike other, much deeper lines that form part of the tube network – has exposed areas behind commercial premises, including restaurants.

But that theory could not explain the smell at Kennington – and, so far, neither can the operator. A TfL spokesperson told the Guardian: “We apologise to customers who may have experienced an unpleasant smell at Kennington tube station recently.

“We are investigating the cause and will work to remove it as quickly as we can.”