Adds Whitbread's response in paragraph 5, shares in 6, details throughout

May 18 (Reuters) - U.S. activist investor Corvex Management on Monday called on Premier Inn owner Whitbread WTB.L to pursue a sale and said it would nominate new directors to the board if the hotel operator refused to act, citing share underperformance and a gap in valuation.

The fund holds roughly 6.1% of Whitbread, according to LSEG data, and in December it urged the group to review its capital strategy, citing a valuation gap after last year's British budget changes saddled the country's biggest hotel operator with higher costs.

"It is imperative that Whitbread immediately retains an independent investment bank and makes a public commitment to conduct a rigorous and comprehensive sale process," Corvex founder and Managing Partner Keith Meister said on Monday.

Meister, a protege of Carl Icahn, in a letter called out Whitbread's "lack of engagement" and demanded it suspend all non-essential spending and leaseback deals while a sale process is underway. He also urged the company to return cash to shareholders through buybacks.

"Whitbread is focused on driving stronger returns for all our shareholders," a company spokesperson said in a statement, highlighting the company's five-year strategy to cut costs.

Whitbread shares were flat at 2,302 pence at 0845 GMT. They have lost 18% of their value over the last 12 months.

In April, the company warned of 3,800 job cuts during its full-year results and said it would shut its 197 remaining branded restaurants and start selling more meals at its hotels.

It also plans to sell 1.5 billion pounds ($2 billion) worth of freehold property to fund growth and reduce net capital expenditures, making it a majority leaseholder for the first time since the Premier Inn chain was founded in 1987.

Corvex criticised the new plan in its letter on Monday and called it "value-destructive."