A warning has been issued over the "patchy and inconsistent" supply of lateral flow tests.
The government has updated the quarantine restrictions so people with covid can leave isolation after seven days if they provide evidence of a negative test.
The chief executive of the Association of Independent Multiple Pharmacies has warned providers have often been unable to find Covid-19 tests to meet the increased demand.
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Leyla Hannbeck told BBC Radio 4's Today programme: "What our members are telling us is that demand for the lateral flow tests is very high at the moment due to the current guidelines around self-isolation.
"Pharmacies are reporting that every five minutes approximately somebody comes in to the pharmacy asking for a test.
"But, unfortunately, because of the issues around supply being patchy and inconsistent, it means that those who come for the test don't always get it, which is very stressful not just for the pharmacy team but for the patient.
"The scale of the problem is huge because the demand is high, because of the current guidelines.
"People are doing the responsible thing by wanting to be tested and we are in the Christmas area of time, and the New Year is just around the corner and people want to be with family and friends."
The UK Health Security Agency (UKHSA) has said nearly 900,000 test kits are being provided per day - which is double the supply seen before December 18 - but Ms Hannbeck said deliveries have been "patchy".
Ms Hannbeck added: "It just simply isn't enough to meet the demand and it's patchy.
"Some days you get one box delivered and other days none. It isn't enough for the pharmacies to deliver to the patients.
"We want to make sure that the message is clear to the government that the supply needs to be consistent."
The UKHSA said tests may be temporarily unavailable on the government website at points throughout the day due to "exceptionally high demand" and urged people not to order more tests before using the ones they have.
A spokesperson said: "During periods of exceptional demand there may be temporary pauses in ordering or receiving tests, to ensure we manage distribution across the system and support changing requirements for LFD and PCR tests.
"Reduced postal schedules also impact on delivery capacity over the Christmas period.
"Rapid tests are available to order directly to people's homes via gov.uk and availability is refreshed regularly throughout the day.
"We encourage people to re-visit the site every few hours if they are unable to order tests as more will become available - and to please ensure they are making use of any tests they already have at home before ordering or collecting more."
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