LONDON — They lived together, traveled abroad on vacation and, for five years, behaved like any other couple. Then one day the man, who called himself Mark Cassidy, disappeared, leading his bewildered ex-partner to eventually discover that she had been living with a married police spy.

The case is one of seven that prompted a formal apology from the British police on Friday for the behavior of undercover officers who used relationships — one nine years long — to gather intelligence and to infiltrate environmental, social justice or other advocacy groups.

In doing so the men had engaged in “long-term, intimate, sexual relationships with women which were abusive, deceitful, manipulative and wrong,” Martin Hewitt, assistant commissioner of the Metropolitan Police, said in a statement.

Offering an unreserved apology, Mr. Hewitt said that the women were “deceived, pure and simple,” adding that “these relationships were a violation of the women’s human rights, an abuse of police power and caused significant trauma.”