Twigg is set to join Nexus on 10 February, ending the agency's seven-month search for a successor to Anna Hooper.
Hooper quit last July after just six months in the role, but has remained as an ad hoc consultant since then (PRWeek, 24 May 2002).
Twigg leaves Harrison Cowley after 12 years. He started in the group's Scotland office before taking control of the agency's 25-strong consumer team in London.
HC chairman and CEO David Heal said Twigg would not be directly replaced.
'We may recruit in other areas rather than trying to find someone like Alan,' said Heal. 'It gives us the opportunity to re-think what we want to do.'
Heal, deputy chairman Paul Kelly and MD Loretta Tobin will share Twigg's responsibilities in the interim.
At Nexus, Twigg will lead a team of over 40, with support from directors Amanda Cryer and Claudine Moyle, who was appointed at the end of last year.
Nexus chief executive Jim Horsley said part of Twigg's responsibility would be to lead an initiative to boost branding work, entitled the Brand Academy.
'We want big brand clients to look at us as an alternative to advertising agencies - we want to demonstrate that our understanding of brands is as good as any ad agency,' he said.
Nexus consumer clients include Tropicana, Muller and Cadbury Trebor Bassett.
Prior to HC, Twigg worked as head of publications at the Milk Marketing Board.