The supply of at-home lateral flow Covid tests will be “constrained” in the coming weeks, the health secretary has said.

In a letter sent to MPs on Wednesday evening Sajid Javid said the public should continue to take tests when coming into contact with vulnerable people or doing risky activities.

But he admitted: “In light of the huge demand for LFDs seen over the last three weeks, we expect to need to constrain the system at certain points over the next two weeks to manage supply over the course of each day, with new tranches of supply released regularly throughout each day.”

Mr Javid said MPs should instruct concerned constituents unable to find kits in the normal way to “see whether their local authority is distributing tests” or to check community facilities such as libraries.

“I would like to thank your constituents for their continued understanding and patience during this unprecedented time,” he said.

The government has placed at-home rapid testing at the centre of its strategy to fight Covid, alongside vaccination – but their growing scarcity is causing problems.

People unable to find a test have to isolate for around 40 per cent longer under government rules, staying at home for 10 days instead of seven.

It comes amid concern about staff shortages across the economy, including in the NHS and on public transport – with Victoria Station in London already closed to its main operator due to a shortage of train crews.

Yet the NHS website has faced rolling shortages and pharmacists in high-demand areas have largely been out of stock.

The government says it has taken steps to increase the supply of tests, with 300 million expected to be delivered in January, up from an original order of 100 million.

The UK government has so far declined to impose new Covid restrictions on people in England, putting it increasingly out of line with countries on the continent, as well as the other home nations.