Standard Life chairman Sir Gerry Grimstone wades in against Brexit

Sir Richard Branson, the billionaire Virgin founder, also added his voice to an “In” vote

'The single market is in the best interests of our customers and clients,' Standard Life's chief executive has said (Getty Images)

City heavyweight Sir Gerry Grimstone threw Standard Life’s support behind the “In” referendum campaign yesterday, warning that Brexit will shut customers out of Europe and spark turmoil for the British economy.

As David Cameron worked furiously to negotiate a new settlement in Brussels, the chairman of the insurance giant warned that leaving the EU single market would sap business confidence, in a move reflecting growing alarm about the threat of Brexit.

Sir Richard Branson, the billionaire Virgin founder, also added his voice to an “In” vote yesterday, saying it would be a “very, very, very, very sad day” if Britons voted to leave.

“I just hope sense will prevail,” he added.

Standard Life played a pivotal role in the Scottish referendum and was widely seen as helping swing the debate towards a “No” vote. It had threatened to move its headquarters from Edinburgh in the event of a “Yes” .

The FTSE 100 giant is keen to air its views on the EU referendum despite stressing it is an apolitical organisation.

Sir Gerry, an influential voice in Westminster due to his time spent advising Margaret Thatcher on privatisations, is a key ally of Mr Cameron.

The Prime Minister has been urging business leaders to speak up more on the threat of Brexit.

“It would be potentially damaging to the UK economy, and therefore to companies such as Standard Life, if the UK were to leave it,” Sir Gerry argued.

“It’s important to use our voice to drive improvements within our industry.”

The insurer is working on a contingency plan if the UK leaves the EU, according to Standard Life’s chief executive Keith Skeoch.

He said the impact on the insurer would involve “deeply, deeply technical stuff” related to currency movements and settling trades.

“It’s in the best interests of customers and clients that we benefit from access to the single market, and we should want to benefit from everything capital markets can bring,” he said. “The critical thing on fund management is that we can continue to operate European vehicles.”

The company receives around 10 per cent of its fee- based revenue from European investors but has a big presence managing money invested in continental European stock markets.

Standard Life is still firing on all cylinders despite the political turmoil and choppy investment markets. Mr Skeoch said its flagship Global Absolute Return Strategies (Gars) fund had slipped recently, but that 88 per cent of its funds were surmounting their performance benchmark hurdles.

“We are in reasonably good shape,” said Mr Skeoch. “Markets driven by geopolitics will remain choppy yet.”

The group attracted a further £6.3bn from customers in the year ending December 2015, helping to drive fee-based revenues to £1.58bn, from £1.43bn last year, thanks to a boost in workplace pensions. Pre-tax operating profits were £665m, up from £608m and the full year dividend was also 7.8 per cent higher at 18.36p.

Standard Life also disclosed a solvency ratio of 162 per cent with a surplus of £2.1bn as it complied with new rules from Europe on capital buffers, called Solvency II

It became the latest insurance company to reveal its capital position after its competitor Prudential said that its solvency position was 190 per cent with a surplus of £9.2bn.