LONDON (Reuters) - Online gambling firm Sportingbet Plc said a 350 million pound ($565 million) offer approach by bookmaker William Hill and GVC Holdings "significantly undervalues" it, but left the door open for a higher bid.

It had received a takeover approach at 52.5 pence per share, consisting of 45 pence in cash from William Hill and 7.5 pence in shares in smaller online betting firm GVC, Sportingbet said on Monday.

"The board of Sportingbet has responded that this indicative offer significantly undervalues the business and its future prospects," it said.

However, it did not say it was rejecting the offer outright.

The statement followed speculation in the weekend press that the board had received a letter containing the joint bid approach, which it had unanimously turned down.

Analysts expect the bidders to come back with a higher offer.

"We believe Sportingbet is worth over 60 pence per share, excluding any bid speculation, and expect Wednesday's full year results to show the business continues to make strong underlying progress," said Panmure Gordon analysts on Monday.

Sportingbet is forecast to report pre-tax profits of around 30 million pounds on sales of 200 million on Wednesday, according to Thomson Reuters I/B/E/S estimates.

Sportingbet has seen its European operations struggle with the economic downturn and a changing regulatory map, but has a strong core Australian business that is attractive to traditional bookmaker William Hill as it expands overseas.

Numis said shareholders should hold out for 90 pence per share, citing the business growth potential and saying it was a chance for the bidder to snap up a bargain while trading was at a low point.

Shares have risen from a low of 26 pence in May to 44 pence just before the approach was announced last month, and have been trading at around the offer level since then.

The bidders have until October 16 to make a firm bid or walk away under UK takeover rules, although this deadline can be extended.

Sportingbet and William Hill both declined to comment further.

(Reporting by Rosalba O'Brien; Editing by Paul Sandle)