England and Scotland ran out of coronavirus home testing kits within hours on Monday, amid a backlog in laboratories.

People were advised to travel long distances to test centres after being told that the daily allowance of home tests had run out.

Callers to the NHS 119 number for Covid-19 were told: “We’re very sorry – the available allocation has already been issued at this time.”

The reason behind the apparent shortage is unclear, but laboratories have been struggling to clear a backlog of tests since the end of July. The Department of Health and Social Care has been contacted for a comment.

The health and social care secretary, Matt Hancock, announced on 28 April that his department was increasing the number of home testing kits available from 5,000 a day to 25,000 a day by the end of that week.

Several people expressed their displeasure on social media after trying unsuccessfully to order a home test on Monday. One man said he had been told to travel to a testing site 36 miles away in Tiverton, south Devon, despite living closer to two testing sites.

Another wrote to Nicola Sturgeon, the first minister of Scotland, saying there were no available slots at regional testing sites “within 100 miles of Glasgow” on Monday.

The UK government does not release figures on how many home testing kits are posted out each day but the number of results recorded from them remains well below 25,000 a day.

In the most recent week for which figures are available, an average of 16,000 results a day were recorded from home testing kits although the number has steadily increased over the past three months.

Home test kits make up about one in four of all coronavirus tests carried out each week in the UK. The self-administered test, in which a swab must be inserted deep into the user’s throat and nose, is far more convenient for those who do not have access to a vehicle or live in remote locations far from the privately run testing sites.

The time taken to produce results from coronavirus tests has risen sharply in recent weeks, owing to an IT failure and an increase in the number of tests being carried out.

Only 14.5% of tests taken at home or at satellite testing centres produced a result within 48 hours in the week to 12 August, compared with a third in the previous week. Nearly one in four of all 114,224 home tests in the week to 12 August took longer than 72 hours to produce a result, compared with one in 25 a month earlier. One in 20 home tests results were not completed at all.

The government said the worsening turnaround times were due to an IT systems failure at one of the laboratories which resulted in a delay to the processing of results. It added that the increase of tests had also resulted in backlogs in some laboratories.

One person said she was told to go to a drive-in test centre after ringing to chase up her results four days after sending off her home test. “I have no car [and] four days on I no longer have symptoms. Plus, I’m meant to be quarantining (which is why I did the home kit in the first place),” she wrote.