(Adds latest COVID-19 numbers)

RIO DE JANEIRO/SAO PAULO, Dec 30 (Reuters) - Brazil will soon weigh emergency-use approval for AstraZeneca's COVID-19 vaccine after Britain gave the green light on Wednesday, as the South American country was forced into making regulatory concessions to speed up its lagging immunization program.

Britain on Wednesday became the first country in the world to give full regulatory approval to the coronavirus vaccine developed by Oxford University and AstraZeneca.

The U.K. approval offers hope to Brazil, which has made the cheap and sturdy British vaccine a cornerstone of its widely criticized vaccine plan.

But it also highlights the problems Latin America's biggest country has had in procuring and utilizing vaccines that will be crucial to ending the world's second-most deadly coronavirus outbreak.

President Jair Bolsonaro, a prominent coronavirus skeptic who has said he will not take any COVID-19 vaccine, is under growing pressure to speed up Brazil's rollout, as regional peers Mexico, Chile and Argentina have already begun immunizations.

On Wednesday, AstraZeneca said Argentina's regulator approved its vaccine for emergency use there. Brazil's government has given a best-case-scenario date of Jan. 20 for AstraZeneca vaccinations to begin.

Brazil's health regulator Anvisa met with AstraZeneca Plc representatives in the morning and said the company's local partners, federally-funded biomedical institute Fiocruz, will file for emergency use authorization, without saying when.

It also said that it had agreed to tweak certain emergency use application requirements that Pfizer Inc had complained were overly onerous.

The federal government's dispute with Pfizer, whose vaccine is already being used in Britain and the United States, led critics to decry needless bureaucratic hurdles just as the virus roars back to life. Brazil recorded 1,194 deaths from COVID-19 on Wednesday, the highest number for one day since Sept. 1.

In theory, Brazil's emergency use authorization allows for fast-track usage of a COVID-19 vaccine among certain high-risk patients. It is a slimmed down version of a full regulatory approval for nationwide rollout.

However, Pfizer's complaints about the process had forced the government onto the defensive on Tuesday, saying it was hamstrung by local laws that only allow it to sign purchase agreements once producers have emergency use authorizations or full authorizations.

However, the government also pledged to improve dialogue with Pfizer.

Its new stance appears to be bearing fruit.

On Wednesday, Pfizer said Anvisa had suggested it could tweak certain requirements to help expedite emergency use approval. Anvisa, in a statement, confirmed its willingness to modify certain requirements.

Pfizer said it would consider whether to apply for emergency use, adding that it continues to regularly submit trial data to Anvisa as part of the full authorization process.

TARGETING JANUARY

Rio de Janeiro-based Fiocruz, which has agreed to import and bottle some 100 million doses of the AstraZeneca vaccine by June and eventually produce the vaccine locally, had previously said it would seek full regulatory approval for the shot on Jan. 15.

On that basis, officials have said nationwide vaccinations could, at the earliest, begin on Jan. 20.

Anvisa's press representatives declined to say when Fiocruz would file its request for emergency use.

Once submitted, Anvisa said it will take up to 10 days to review the application, adding that the ongoing submission of late-stage trial results would help to accelerate the process.

AstraZeneca and Fiocruz did not immediately answer questions on plans and timing for seeking regulatory approval.

Earlier in the day, AstraZeneca said it was working to offer its COVID-19 vaccine to Brazilians as soon as possible, but made no mention of seeking emergency use approval. (Reporting by Eduardo Simoes in Sao Paulo and Ricardo Brito in Brasilia Writing by Gabriel Stargardter Editing by Brad Haynes and Alistair Bell)