He is thought to have separated from his wife about a year ago. Some time afterwards, he started a relationship with Mr L’Roubi, 42, whose designs are worn by celebrities such as Claire Rutter, the soprano, and Wilnelia Forsyth, the wife of the Strictly Come Dancing host Bruce Forsyth and a former Miss World.
The British-born designer, whose parents are Somalian, is an opera enthusiast and is also said to enjoy hosting evenings of baroque music and champagne.
His website lists ecclesiastical design commissions for clergy at St Paul’s Cathedral, evening wear for orchestras, and sporting wear for the gunmaker Holland & Holland.
Mr Lagrange became a director of Mr L’Roubi’s company, Asked For Designs, in April and the pair were photographed arm in arm at this summer’s Venice Biennale.
Mr L’Roubi confirmed at the weekend that they are a couple. Mr Lagrange said: “I only met Roubi long after our divorce decision. My relationship with him is indeed of a romantic nature. My divorce reflects the reality of a separation that has happened a long time ago. Our divorce has been the most amicable one, with love and respect from all parties.”
But experts said the former banker could have to pay his wife up to half of his fortune in what would be one of Britain’s largest divorce settlements.
The record was set in July by the former wife of the Russian oligarch Boris Berezovsky, 64, whose payout was estimated at £220 million.
The Lagranges’ links to the Conservative Party caused embarrassment for David Cameron after GLG was forced to declare that it had been “short-selling” shares in Bradford & Bingley.
The practice was blamed for the collapse of Lehman Brothers and the near failure of HBOS and Merrill Lynch. It emerged months later that Mrs Lagrange had given £50,000 to the party and her husband was a member of the Leader’s Group, a donors’ club which had access to the leader of the opposition.
Mrs Lagrange gave another £100,000 to the Tories before the election. Mr Lagrange bought the house in Kensington Palace Gardens for £19 million in 2004.
He sold the grade-II listed property for a profit of £71 million after renovation work that included an Olympic-sized swimming pool.