Barclays has waded into the debate over Britain's membership of the European Union, claiming a Brexit would leave customers worse off.
The chairman of the banking giant John McFarlane said the UK and international economy was "enhanced" by Britain remaining a member of the EU.
His comments came as insurer Legal & General said it had not taken a view on whether Britain should stay in Europe, but said the current economic case for leaving was "unproven".
Barclays' stance will be seen as a boost for the UK prime minister David Cameron who is campaigning for Britain to remain in the EU ahead of the referendum on Thursday June 23.
Cameron has already won support from corporate heavyweights, such as Shell, BAE Systems, BT and Rio Tinto after the chairmen and chief executives from around 30 FTSE companies urged Britain to stay in Europe.
Mr McFarlane said it was in the interest of Barclays' "customers and clients for the UK to remain in the EU".
He said the bank was also pressing the case to stay in because of the key services it offers European companies from London.
He added: "More importantly, we are heavily reliant on a successful UK domestic and international economy and feel this is enhanced through the UK's membership."
In a statement, Legal and General said it would keep the referendum "under review as more and better data and analysis emerges".
However, it said an exit vote would lead to a lengthy period of renegotiation, creating "uncertainty for markets and the broader UK economy" in which it operates.
It added: "As one of the largest investors in the UK, we will be actively listening to companies we invest in, who will be assessing the potential impact for themselves."
Fears that Britain may head for the European exit door has caused the value of sterling to fall to seven-year lows in previous market sessions.
Researchers at HSBC said the value of the pound against the dollar could plummet by as much as a fifth if market uncertainty around Brexit persists.