The Rt Rev Richard Chartres, who retired in March this year, will sit as an independent crossbencher.
Lord Chartres, 70, was flanked by supporters Baroness O’Neill of Bengarve and Lord Luce as he swore the oath of allegiance to the Queen.
As bishop of London, he was one of the 26 senior members of the Church of England, known as the Lords Spiritual, entitled to sit on the red benches of the unelected upper house. Others include the Archbishops of Canterbury and of York.
The former bishop of London has returned to the House of Lords after his appointment as a life peer.
A shrewd negotiator knows the other side’s position as well as she knows her own. Theresa May’s problem is that even if she does, she knows that some of her senior Ministers are opposed to the EU’s expectations.