Lord Archer had an affair with Ms Andrina Colquhoun during the time he faced allegations that he had had sex with a prostitute, his former secretary said yesterday.
Ms Angela Peppiatt told his Old Bailey perjury trial that she was part of a cover-up to keep his reputation "whiter than white" leading up to a libel trial.
She said his personal life had been put through intense scrutiny at the libel trial when Lord Archer successfully sued the Daily Star after denying having sex with Ms Monica Coghlan, and was awarded £500,000 damages in July 1987.
But Ms Peppiatt (56) said Ms Colquhoun's liaison with the best-selling novelist was blurred by her being involved in projects such as flower-arranging and refurbishing a bathroom at his penthouse near the Thames.
Ms Peppiatt said the couple had pet names for each other. He was known as Moon and she as Roonett.
At one stage Lord Archer bought two sets of jewellery from the same shop, one for his wife, Mary, and one for Ms Colquhoun.
Asked why she had produced invoices for the bathroom dating from November 1986 to May 1987, Ms Peppiatt said: "I took these original documents because I suppose it was because Jeffrey was saying to the outside world once again that he was whiter than white, and was not having any extramarital affairs, and - under our very noses - we knew what was going on."
Lord Archer always denied having had sex with the prostitute but resigned as deputy chairman of the Conservative Party after the News of the World printed a report that he had arranged for the woman to be given money at Victoria Station.
Ms Peppiatt said she had taken invoices and documents, some of which did not belong to her, in order to prove what she knew, particularly after his resignation.
Lord Archer denies four counts of perverting the course of justice, two counts of perjury and one of using a diary as a false instrument.
His former friend, Mr Ted Francis (67), a television producer from Surrey, denies one charge of perverting the course of justice.
Mr Roy Amlot QC, for Mr Francis, asked Ms Peppiatt what in her view had been the relationship between Ms Colquhoun and Lord Archer in September 1986.
Ms Peppiatt replied: "She was his mistress, absolutely, his girlfriend."
The hearing continues today.