Home Office mandarins are demanding an apology from Suella Braverman for offering “praise [to] staff in private only to attack them in public” after an email in her name claimed civil servants had blocked attempts to curb migration. A letter sent to the department’s most senior civil servant has accused the home secretary of “an outrageous attack on the integrity and impartiality of the thousands of Home Office staff” and says she must apologise. It adds to evidence of anger within the Home Office at an email signed by Braverman which blamed “an activist blob of leftwing lawyers, civil servants and the Labour party” for blocking laws which curb “illegal” migration. Braverman insists she neither saw nor sanctioned the email before it was sent to thousands of party activists. Wynne Parry, the Home Office rep of the FDA union, sent the letter to Sir Matthew Rycroft, the Home Office permanent secretary, on Wednesday. Parry, whose union represents senior grade civil servants, said the home secretary’s explanation that she did not sanction the letter is not enough to calm “furious” staff – and she must still say sorry. The letter sent to Rycroft says: “You will already be aware of the great upset this cowardly attack has caused to Home Office staff. “It is vital that the home secretary makes a full apology to staff for these comments. “It is not acceptable to praise staff in private only to attack them in public when civil servants have no recourse to defend themselves.” The email sent under Braverman’s name to Conservative party members said: “We tried to stop the small-boat crossings without changing our laws. But an activist blob of leftwing lawyers, civil servants and the Labour party blocked us.” Speaking to ITV’s Robert Peston Wednesday evening, Braverman said: “I didn’t write that email. I didn’t see it and it was an error that it was sent out in my name.” There is evidence of growing anger within the Home Office at Braverman’s comments as well as her language while referring to asylum seekers. In a leaked Home Office staff Q&A after the email was sent, one civil servant wrote: “I have never been so embarrassed and ashamed to work for the department I once loved. Time to move on!” Another complaint seen by the Guardian questions Braverman’s description of the numbers of people seeking refuge in the UK as an “invasion”. “This is disgusting language and does, I believe, encourage the far right in their activities,” it said. In an attempt to repair relations with civil servants, Braverman sent an email to Home Office staff lavishing praise on them for their help with the illegal immigration bill. “I can’t possibly name you all, but I have received outstanding support from the whole department,” Braverman wrote. Braverman has been accused of possibly breaching the ministerial code because of the email. The head of the FDA union, Dave Penman, wrote to Rishi Sunak on Wednesday asking him to investigate. It comes as Rishi Sunak is preparing to go to France in an attempt to shore up the illegal migration bill, which Braverman presented to parliament on Tuesday. Pressure continues to mount on Sunak’s government after widespread condemnation of the proposals from national and international bodies. The United Nations refugee agency warned that the bill risked “extinguishing the right to seek refugee protection in the UK”. Braverman’s office declined to comment further.