The UK's Covid vaccination rollout will receive an extra £1.65bn in the Budget to help it reach its target of offering a first dose to every adult by 31 July.

The chancellor will also announce £22m of the programme's existing funding will be used in a trial to see if mixing different vaccine doses works.

Over 20 million people in the UK have had a first dose and NHS England is now asking 60 to 63-year-olds to book jabs.

Rishi Sunak said it was "essential we maintain this momentum".

He added: "Protecting ourselves against the virus means we will be able to lift restrictions, reopen our economy and focus our attention on creating jobs and stimulating growth."

The trial on vaccines will also investigate whether a third dose can be effective. The government is aiming to offer all adults a first jab by the end of July.

Meanwhile, nine out of 10 people aged 65 and over have had their first Covid vaccine dose, according to the NHS in England.

The Treasury said the chancellor is also expected to outline improvements to the UK's ability to respond to new Covid variants in his Budget announcement.

Mr Sunak will pledge to put £28m from the Vaccine Taskforce's existing funding into expanding the UK's vaccine testing capability and ability to quickly acquire samples of new coronavirus variants.

And £5m will be used by the Centre for Process Innovation in Darlington to acquire a "library" of Covid vaccines to use in its work against different variants.

It come as a coronavirus "variant of concern" first detected in Brazil has been found in the UK, with three cases detected in England and three in Scotland.

In England, officials are still trying to track down one of those who tested positive for the new variant. The first two cases were from the same household in South Gloucestershire, with a history of travel to Brazil, but the third is not linked, Public Health England (PHE) said.

There are also concerns vaccines may not be as effective against the Brazilian variant - but NHS England's Prof Stephen Powis said vaccines could be "rapidly adapted".

The latest government figures show 20,089,551 people in the UK have now had a first dose, while 796,132 have had a second.

Prime Minister Boris Johnson called it a "huge national achievement" and said "every jab makes a difference in our battle against Covid".

The government met its pledge to offer a jab to everyone in the top four priority groups in the UK, including the over-70s and frontline health and care staff, by mid-February.

From Monday, nearly two million people aged 60 to 63 in England will start receiving letters inviting them to book their vaccine.

The next target is to offer a first dose to all over-50s by 15 April, as well as people aged 16-64 with certain underlying health conditions and unpaid carers for disabled and elderly people.

After that, people will be prioritised by age group.

Restrictions in England are set to be fully lifted by 21 June at the earliest, with all pupils set to return to schools from 8 March.

Speaking on the BBC's Andrew Marr Show on Sunday, the chancellor said he "is preparing a Budget that provides support for people" as Covid lockdown rules are eased.

Mr Sunak confirmed he would provide help up to then but added that he wanted to "level with people" about the "shock to the economy" caused by Covid.