The PRA is responsible for prudential regulation and supervision of banks, building societies, insurers and major investment firms.
Mr Prettejohn has worked for L&G since 2010 and also works as chairman of Brit Insurance Holdings, a role he took up in 2011. Prior to this he worked as chief executive of Prudential UK and Europe.
Mr Prettejohn will be the fourth independent director on the board, joining Iain Cornish from Yorkshire Building Society, Rosalind Gilmore from Lloyds of London and Charles Randell from corporate law firm Slaughter and May who were appointed in March.
Other members are chairman Sir Mervyn King, deputy governor for financial stability Paul Tucker, PRA chief executive Andrew Bailey and Financial Conduct Authority chief executive Martin Wheatley.
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Sir Mervyn said: "I very much welcome Nick's appointment and I am sure he will play an invaluable part in establishing the Bank's new judgement-based approach to supervision in the UK."
He will take up his role at the PRA, the regulator for banks and insurance companies, on 10 June. His remuneration will be £77,520 per year.
Commenting on the appointment, Otto Thoresen, director general of the Association of British Insurers, said: "Having insurance expertise at the top table of the regulator will help ensure proportionate and sensible regulation of the industry, so we welcome Nick's appointment.
"As an ex-board member of the ABI and with his extensive London market experience, Nick is an excellent choice for this important role."

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