British families stranded in India have said they feel abandoned by the UK government amid reports of shortages of food, water and medicines as the entire country was locked down due to the coronavirus pandemic.

Narendra Modi, the Indian prime minister, announced on Tuesday that the entire population of 1.3 billion people would be banned from leaving their homes for three weeks as all domestic and international flights were grounded.

Thousands of UK citizens are thought to be stuck in India with the only return flight currently scheduled for the early hours of Wednesday morning from Goa.

A group of six British citizens, aged between 61 and 83, who have tested positive for coronavirus are among those stranded. On Tuesday they were facing a fifth night in what relatives described as “appalling, unsanitary conditions” in a hospital in Kerala on the country’s southern coast.

Katherine Webster, from Newquay in Cornwall, said her parents were among the six tourists being held in “filthy” hospital isolation units infested with rat droppings and were without bedding, toilet paper, soap, towels and only had very basic food rations.

The 44-year-old said her parents, Liz Lawson, 75, and Nairn Lawson, 76, had tested positive for Covid-19 and were currently asymptomatic, but her mother had developed another health condition since entering the state hospital.

Webster said she had been desperately trying to get the Foreign Office to move the six into safer conditions. “They’re living in terrible conditions. We’re just worried the conditions they’re living in are going to make them so ill. They’re finding it really, really difficult. It’s going to have a massive impact on their mental health I think.”

A Foreign Office spokeswoman said: “Our staff are supporting a group of British people currently undergoing medical treatment in India and are in touch with the local health authorities.”

Facebook Twitter Pinterest A man crosses an empty expressway in New Delhi. Photograph: Altaf Qadri/AP

One 62-year-old woman from the Midlands, who is trying to fly home from Goa with her 62-year-old partner, said she had contacted the Foreign Office 10 to 20 times but only received a “copy and paste” message in response with little information. The woman, who did not want to be named, said her diabetic husband was running out of insulin and needles and local pharmacies, which were only allowed to open for a few hours each morning, were refusing to sell them to her.

“People are getting so, so worried. I can’t tell you how frightening this is for people and we’re getting nothing from the government. Someone’s got to listen to us because someone’s going to get killed or end up dead,” she said.

Rupinder Gill, 49, from Bedford, said her family of seven had been unable to leave a house in the Punjab region for three days and that she was “extremely worried” that her 81-year-old mother-in-law would soon run out of medication.

Marineder Hanif, from Derby, said he was deeply concerned about his mother, aunt and uncle, aged 64, 68 and 67, after they were robbed during an eight-hour drive from Delhi to relatives in a Punjabi village, which is now closed to visitors due to the lockdown.

“With everything that is going on, it’s very scary. I am worried most about my auntie. She had heart and respiratory problems and diabetes. It is just a horrible time,” he said.

A Tui flight from Goa to Manchester is chartered for the early hours of Wednesday morning local time, yet many Britons said they were unable to book on to the flight or had been ordered by local police and taxi drivers not to travel to the airport amid the nationwide lockdown.

The foreign secretary, Dominic Raab, came under fire on Tuesday from his Conservative party colleague Caroline Nokes, who said the government’s assurances felt “empty” to Britons stranded abroad.

Nokes, a former Tory minister, questioned what help the UK’s “global network” was offering, referring to Britons having to sleep rough on the streets of Caracas in Venezuela.

The Foreign Office has advised the estimated 1 million Britons overseas to return to the UK immediately as they may not be able to get commercial flights within days. The Guardian has spoken to scores of UK citizens facing an increasingly desperate situation, including Britons stranded in Peru, Thailand, the Philippines, Argentina, Columbia, Venezuela and New Zealand.