It says: "We hope you agree that it would send a awful signal if a convicted felon deemed inadmissible to the United States such as Yaxley-Lennon were allowed to travel to your country and speak before a prominent audience despite his conviction for previously entering the United States illegally".
Far-right Mr Robinson, real name Stephen Christopher Yaxley-Lennon, is said to be waiting for authorisation under the US ESTA system before making the trip to Washington to speak at an event.
The letter states: 'We hope you agree that it would send a awful signal if a convicted felon deemed inadmissible to the United States such as Yaxley-Lennon were allowed to travel to your country and speak before a prominent audience despite his conviction for previously entering the United States illegally.
Mr Robinson is also planning a tour of Australia and has previously claimed "lies" are being spread about him to stop him travelling to other countries.
Robinson was convicted of contempt of court over the summer, but this ruling was later dissolved. His conviction was later quashed.
The online payment platform PayPal has closed down the account of right-wing British activist Tommy Robinson and frozen the money that he had there for what they call promoting "hatred and violence".
Online petitions calling on finance firms to sever links with him have gained thousands of signatures.
In a statement, Paypal said it could not comment on individual customers but added that it regularly reviewed accounts to ensure their use aligned with its acceptable use policy.
The Attorney General's Office has said all the material was being looked at "afresh" before a decision was made on whether to refer Robinson to the High Court for contempt.
'The Government and establishment can see I have public support, they can see I have the ability to fight back'.
Robinson's right-wing rhetoric has garnered a lot of support both in the United Kingdom and the U.S., but has also set him at odds with leftist activists, who have asked PayPal to cut off his account.
PayPal said: "Striking the necessary balance between upholding free expression and open dialogue and protecting principles of tolerance, diversity and respect for all people is a challenge that many companies are grappling with today".