Almost half of David Cameron's most senior business advisers have not signed his pro-EU campaign's letter calling for an In vote in the referendum.

Nine of the 20 members of his Business Advisory Group, which provides the Prime Minister with "high level advice" on "critical business and economic issues facing the UK", have not added their names.

They include executives from the bank Santander, financial services company Legal and General, the accountants Ernst & Young and property firm Land Securities.