Bolton, 54, was already facing calls for his resignation after he left his wife for 25-year-old Jo Marney.
Now The Mail on Sunday can reveal shocking messages in which Marney says that Prince Harry's "black American" fiancee will "taint" the Royal Family with "her seed" and pave the way for a "black king".
Marney also says that she would never have sex with "a negro" because they are "ugly".
She was immediately suspended from Ukip last night after The Mail on Sunday alerted party chiefs to her grossly offensive comments, reports Daily Mail.
Our revelations come just days before Bolton is due to face a crunch meeting over his own future with Ukip following outrage over his new relationship.
The former Army officer left the mother of his two young children, 42-year-old Tatiana Smurova, for Marney, whom he met shortly before Christmas.
Ukip's chairman, Paul Oakden, has said he realises the "strength of feeling on this" among grassroot members.
If Bolton is ousted by the party's ruling National Executive Committee on Thursday, it would leave the party having to elect its fifth leader in barely 18 months.
His position looks increasingly shaky as there is no recent precedent for the partner of the leader of any political party to have been suspended from that party.
One Ukip figure even predicted that the scandal could signal the end of the party, which has battled serious financial problems since Nigel Farage stepped down as leader in the wake of the EU referendum.
In Marney's messages, which she sent to a male friend just three weeks before starting her relationship with Bolton, she sneers at Harry's engagement to Los Angeles-born Markle – who will marry Harry on May 19 – on the grounds that she is a "black American".
Marney also describes Markle, whose mother is black and whose father is white, as "a scrubber" and a "gender equality t***" who is "obsessed with race" and a 'dumb little commoner'.
Alluding to ethnic minorities in general, Ms Marney writes that they are "pushing their way to the top slowly slowly. Next will be a Muslim PM. And a black king".
The ex-model – who has remained in touch with the friend she texted since starting her relationship with Mr Bolton – tells him that she would not have sex "with a negro".
When the friend asks her what is wrong with black people, she replies: "Ugly. Not my thing. Just personal taste. They don't turn me on."
During the exchange, Marney – who is from Kent and describes herself as an actor and music journalist – also complains that there is "an immigration problem", saying: "This is Britain. Not Africa".
After her friend accuses her of being racist, she writes: "Lol so what... Not wanting other races and cultures to invade your own culture doesn't mean I hate their race. Just means I don't want their cultures invading mine."
After being confronted with her texts by The Mail on Sunday last night, Marney apologised for her comments which she claimed had been "taken out of context".
In a statement, she said: "I apologise unreservedly for the shocking language I used. The opinions I expressed were deliberately exaggerated in order to make a point and have, to an extent, been taken out of context. Yet I fully recognise the offence they have caused.
"No offence was intended and, again, I apologise unreservedly for any such offence or hurt that my messages have caused to members of the public, members of Ukip my friends, family and loved ones.
"I have disappointed them all and let myself down. I cannot sufficiently express my regret and sadness at having done so."
Oakden said: "In light of messages that have been brought to my attention, I have taken the decision to immediately suspend her party membership pending an investigation. Ukip does not, has not and never will condone racism."
Since news of his relationship broke, Bolton has repeatedly insisted that he will not resign as he is convinced another leadership contest would "cripple the party".
He claimed that his marriage to Smurova was already over when he met Marney, and has said: "I have no doubt I'm going to come out of this, and Jo and I are going to come out of this as a couple, stronger than we went into it."
Before the racist texts emerged, Oakden said Bolton and the party were "keen to work collectively in finding a resolution that best protects the future prospects of Ukip" following the scandal over the relationship.
Farage quit after the 2016 referendum, saying he had achieved his political ambition. He was followed as leader by Diane James, who lasted just 18 days after feuding with the party's "old guard".
Farage then returned briefly, to be succeeded by Paul Nuttall in November 2016. Nuttall resigned after seven months following the Election, when Ukip's polled less than 2 per cent.
Steve Crowther took over until Bolton was elected in September. He has failed to lift Ukip's poll ratings above 4 per cent.