Wimbledon announced last month that it had lifted a ban that prevented athletes from the two countries playing last year following Russia's invasion of Ukraine in February 2022.
Club chairman Ian Hewitt on Tuesday said the U-turn had been the hardest decision of his four years in the job.
"This was an incredibly difficult decision," he told reporters. "It was a combination of factors and developments since last year which led to our decision."
Wimbledon and the other British grasscourt tournaments stood alone in barring Russian and Belarusian players such as former world No.1 Daniil Medvedev and Victoria Azarenka last year.
The decision resulted in ranking points being stripped from the events - a considerable blow to Nick Kyrgios who gained no points from reaching the final, and the UK's Lawn Tennis Association being hit with huge fines by the men's and women's Tours (ATP and WTA).
To compete this year, Russian and Belarusian players must agree to compete as neutrals, not to make statements supporting the war or the regimes involved and not to receive funding from either nation or state-supported businesses.
"The development of the wording of the declarations is obviously based on government guidance, working with the Tours, the ITF and our partners at the LTA," AELTC chief executive Sally Bolton said.
"As soon as we made the announcement players were able to sign the declarations and they are able to right up until the entry deadline. I won't share the details of who but a number of players have already signed. They fully understand what they are signing up to."
Wimbledon's U-turn attracted some criticism with Ukraine's Foreign Minister Dmytro Kuleba calling it "immoral".
Bolton said she had spoken to Ukrainian players at the time of the announcement and that the majority had understood the situation.
Hewitt re-emphasised Wimbledon's "condemnation of Russia's invasion of Ukraine", and said the tournament would continue to support players and refugees from the country.
Wimbledon will meet all of the accommodation costs of Ukraine players during the tournament and all of the build-up events in the British grasscourt season, while £1 ($A1.9) will be donated to Ukraine relief efforts for every ticket sold -- amounting to around £500,000 ($A930,000).
In a separate development Polish carrier LOT confirmed it refused boarding to Russian player Vitalia Diatchenko, ranked 250th in the world, in Cairo.
"I slept at the airport, I was treated like a third class citizen (because of my nationality), spent a few thousand euro," Diatchenko wrote on social media.
The 32-year old, who said she was subsequently prevented from buying a ticket by German airline Lufthansa due to European Union visa regulations, had been attempting to travel to a tournament in Corsica via Warsaw and Nice.
LOT cited restrictions introduced by Poland's interior ministry during the COVID-19 pandemic and updated in 2022 following Russia's invasion of Ukraine.
