Speeding up lockdown risks leaving the country “flying blind” to the risks of another surge in cases, the government’s chief scientific adviser has warned.

Sir Patrick Vallance told a group of MPs the five-week gap between each step of easing restrictions was needed to evaluate the impact on infections and transmission.

He was echoed by Professor Chris Whitty, England’s chief medical officer, who said “big blocks of risk are being taken together,” every time measures are relaxed.

Prof Whitty added models demonstrated that “if you open up too fast, a lot more people die”.

“If we unlock too quickly we would get a substantial surge whilst a lot of people are not protected,” he said.

The first step in Boris Johnson’s four-part plan to ease England’s third national lockdown began on Monday.

Schools and colleges reopened across England and care home residents were allowed to welcome a single, nominated visitor.

The next steps are expected to come on 29 March, 12 April, and the 17 May.

The prime minister hopes to be able to lift the majority of COVID rules by 21 June, but this is dependent on cases, deaths and hospital admissions continuing to fall so the dates are the earliest any of the stated rules could be lifted.

Each step of the four-part roadmap out of lockdown is separated by five weeks – with four weeks to analyse the latest COVID data, and one week to give notice to businesses.

The hope is to spread out the easing process to avoid a further surge of the virus and avoid a return to the previous system of regional tiers.

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