Wall, who has for the past year acted as adviser to the Archbishop of Westminster, Cormac Murphy-O'Connor, joins on 1 July on a three-day-a-week contract. He will report jointly to the head of H&K's global PA practice Elaine Cruikshanks and EMEA chairman and CEO Andrew Lawrence, who described Wall's hire as 'a major coup'.
Cruikshanks said: 'With uncertainty on the future of the EU, trusted advisers with first-hand experience to guide clients through the maze are hard to come by.'
According to the agency, Wall will 'leverage H&K's capabilities in Brussels, London and other European Union capitals'.
A keen backer of the European constitution, Wall advised Tony Blair on EU matters for four years. He quit the post in 2004, ending a 30-year career in central government.
During that time he has been a private secretary to former prime minister John Major, and principal private secretary to former foreign secretaries Geoffrey Howe, Major and Douglas Hurd.
Wall entered the Foreign & Commonwealth Office as a press officer in 1974.