A man, in his 60s, was taken to hospital with smoke inhalation but his condition is not life-threatening, while another man was treated at the scene.
A spokesperson for the Metropolitan Police told the Standard: “The incident is being treated as arson.
“A man, aged 60s, suffered smoke inhalation. He was taken to hospital by LAS where his condition was not life threatening.
“There have been no arrests at this time. Enquiries continue.”
The investigation comes as witnesses described on social media the moment a group of “yobs” - identified as white youths wearing hoodies - launched the attack.
Chris Hobbs wrote: “My local newsagents victims of a life threatening arson. Yobs threw in petrol followed by a match yesterday evening. Great Sri Lanken [sic] brother & sister couple run it.
“Luckily the female cousin working there was able to flee out of the back door. The flames were fierce & fire appliances from Ealing and Southall attended. Police reassurance patrols in the area. White youths in hoodies responsible.”
Meanwhile, a Twitter user identified as Vasa shared a photo of the aftermath of the blaze, saying: “Pretty dramatic scenes in Hanwell as one of the stores in the older part of the street caught fire.
“Ppl [sic] could only stand aside & witness fantastic response by the local fire brigade & other services.”
Six fire engines and around 40 firefighters were called to a fire at the shop, with part of the ground floor and first floor flat were damaged, London Fire Brigade confirmed.
The brigade’s 999 control officers took 10 calls to the blaze.
Station commander Dean Wilkinson, who was at the scene, said: “Crews worked incredibly hard to quickly bring the fire under control.
“Roads were closed and buses were diverted whilst firefighters made the scene safe.
“This is a timely reminder to test your smoke alarms. There were no working smoke alarms fitted inside the property.
“Having working smoke alarms is so important in helping to detect fires and save lives. They can provide time to escape.
“As a minimum you should have smoke alarms on every floor - in the hallways and the rooms you use the most, plus a heat alarm in the kitchen.”
Firefighters were called at 7.39pm and the fire was under control by 8.23pm.
Crews from Southall, Acton, Ealing and Heston fire stations attended the scene and roads were closed while the fire was being put out.
Meanwhile, the Hanwell community have launched a crowd-funder which people can donate towards to help the shop owners “rebuild their livelihood”.
Claire Mathieu, who started the GoFundMe page, said: “Let's support the lovely owners in rebuilding their livelihood and show them the wonderful community we have here in Hanwell.”
It had already reached £445 in donations by 5pm on Tuesday.
Anyone with information relating to this incident should call police on 101 or Tweet @MetCC quoting 7038/7JUN. To remain anonymous call Crimestoppers on 0800 555 111 or visit Crimestoppers_uk.org.