Pharmaceutical company Roche, a major supplier of reagents, said there had been a 'delay in the dispatch of some products' following a shake-up of its supply process.
The shortfall has left laboratories with limited processing capacity - and GPs in parts of the UK have been told only to request tests in the most urgent cases.
One senior GP told GPonline that the process has reduced by around 70% the number of tests GPs are able to ask for - compounding problems for patients with chronic conditions whose care has been affected through the peak of the COVID-19 pandemic.
Lincolnshire LMC medical secretary Dr Kieran Sharrock said the problem was adding significantly to practice workload - and could increase the risk of GP burnout at a time when pressure on practices has already spiked beyond pre-pandemic levels.
Dr Sharrock said his understanding was that the shortage was 'a national problem' - and that he had spoken to colleagues in neighbouring areas who were also affected.
'The laboratories are telling us that if we send too many tests they won't be able to process them,' he said. 'So it's only the most urgent that we can request.'
He said that general practice was already requesting fewer tests than in pre-covid times, but that a majority even of this reduced level of testing would now be on hold.
'Of the tests we are doing, probably 70% are routine, 30% are more urgent - so we are probably having to cancel 70% of tests for patients - and those will have to be rearranged for a later date.'
He added that practices had not been told how long the problem would last and that it was 'worrying'. Dr Sharrock said: 'If this is weeks to months that really will create a lot of anxiety for patients and doctors who can’t monitor their patients adequately - as well as the work for practices who have to book and re-book.
'It does feel a little bit like we’re being kicked while we’re on our knees. GPs are working exceptionally hard now and workload is significantly higher than pre-COVID. Now the extra burden in terms of deciding what to prioritise feels like a kick in the guts.
'Decision fatigue sets in earlier when you can’t see patients face-to-face - we're seeing more patients with less information. And on top of that to have one less thing to help with decision making. This will increase risk of burning out - GPs are literally having to think harder for every patient they see.'
A spokesperson for Roche said: 'We deeply regret that there has been a delay in the dispatch of some products and apologise to any of our customers who have been impacted.'
The spokesperson said staff at its national distribution centre were 'working exceptionally hard, day and night, to resolve this issue as soon as possible'.
He added: 'As well as extending working hours, we have recruited extra staff and, where they can, our dedicated teams on the ground are working with customers to distribute products and minimise service disruption. We will continue to provide regular updates to our customers and we are doing everything possible to return to routine operations.
'We are prioritising the dispatch of COVID-19 PCR and antibody tests and doing everything we can to ensure there is no impact on the supply of these to the NHS. Roche pharmaceuticals are not affected.'
The company began distributing goods from its national centre only last month and has said delays in dispatch of some items were linked to the move.
A spokesperson for Lincolnshire CCG said: 'We are urgently seeking clarity from our partners on the issues that have caused the shortage of reagent supply, to understand what measures can be put in place in order to be able to support our GP practices and patients.'
NHS England has yet to reply to a request for comment.