The policy has previously applied to those under 30 but the age threshold has been raised after the Medicines and Healthcare products Regulatory Agency reported new figures on clots linked to the vaccine.
The advice comes from the Joint Committee on Vaccination and Immunisation (JCVI) who is extending their guidance from last month where people under the age of 30 were given the same advice.
Last month, people under the age of 30 were told they will be offered an alternative Covid-19 jab to the AstraZeneca vaccine.
The regulator stressed that this was not proof that the jab was the cause of the clots but conceded that the link was getting stronger.
The new guidance comes despite the JCVI and the Medicines and Healthcare products Regulatory Agency (MHRA) insisting there are no new safety concerns.
Following research last month, the committee concluded that the benefits of the jab outweigh the risks, but as people under 30 are at less risk of coronavirus they should be offered an alternative jab.
That advice has now been extended to people under the age of 40.
In Europe, a review by the European Medicines Agency’s safety committee has concluded that “unusual blood clots with low blood platelets should be listed as very rare side effects” of the Oxford/AstraZeneca vaccine.
The European Medicines Agency reiterated that the AstraZeneca vaccine had been “proven to be highly effective” and that vaccination as a whole is “extremely important” in the fight against Covid-19.
There are thought to be no supply issues with offering the vast majority of younger people alternatives to AstraZeneca.
Professor Wei Shen Lim, Covid-19 chairman for JCVI, said: “Safety remains our number one priority.
“We have continued to assess the benefit/risk balance of Covid-19 vaccines in light of UK infection rates and the latest information from the MHRA on the extremely rare event of blood clots and low platelet counts following vaccination.
“As Covid-19 rates continue to come under control, we are advising that adults aged 18 to 39 years with no underlying health conditions are offered an alternative to the Oxford/AstraZeneca vaccine, if available and if it does not cause delays in having the vaccine.
“The advice is specific to circumstances in the UK at this time and maximises use of the wide portfolio of vaccines available.
“The Covid-19 vaccines have already saved thousands of lives and the benefit for the majority of the population is clear – if you are offered the vaccine, you should take it.”