Rolls-Royce Holdings  (RYCEY) , (RYCEF) CEO Warren East has penned a letter its 50,500 employees warning that breaking with the European Union would increase costs and delay the aircraft engine maker's investment decisions, the BBC reported, as concern mounts that U.K. voters will opt to leave the European Union when they cast their ballots in the referendum on June 23.

"We're making investment decisions all the time about where to place different parts of our operations and uncertainty created by Brexit puts a lot of those decisions on hold," East told the BBC in a later interview.

The appeal to employees comes after BT  (BT) chairman Mike Rake and CEO Gavin Patterson wrote a letter to staff  on Monday outlining the merits of remaining with the EU.

Meanwhile, U.K. Chancellor of the Exchequer George Osborne, one of the leading campaigners for the U.K. to remain in the EU, warned that a vote to leave would force the government to cut back on public spending, including on healthcare, and increase taxes. He said it would leave the U.K. with a GBP30 billion ($42.6 billion) budgetary black hole.

The Economist Intelligence Unit, a London-based think tank, has also weighed in, estimating that a break with the EU would mean real U.K. GDP would be as much as 6% below its baseline forecast by 2020 in the event of a "No" vote.

"This economic pain would be coupled with political instability, as significant doubts emerge about government cohesion," the EIU said in its report, adding that "the impact will be serious, and prolonged."

The warnings come as the most recent poll conducted by YouGov for The Times showed a clear lead for a "Brexit" with 46% of respondents saying they would vote to leave, 39% to remain, and 11% undecided.

Rupert Murdoch-owned The Sun, the most widely read tabloid in the U.K., has also urged readers to vote to exit the EU.

Despite the pro-Brexit panic, the pound on Wednesday regained some of the ground it has lost in recent days and was up 0.47% against the dollar at $1.4181.  The FTSE 100, along with mainland European indices, was also up on Wednesday, recently trading 0.85% higher at 5,975.26.