The takeover tussle for Aga Rangemaster heated up as American suitor Middleby urged the cooker firm's investors to be "wary" and rebuff Whirlpool's rival approach.

Illinois-based Middleby said Aga shareholders should "consider the motivations of the approach by Whirlpool" after it has sought to gatecrash the deal at the last minute with a rival approach.

Aga revealed earlier this week it was in talks with US electronic appliances giant Whirlpool over a possible offer after receiving an approach and has opened its books to the suitor.

It came just a week before Aga shareholders are due to vote on Middleby's £129 million deal, with Aga set to officially seal the takeover on September 16.

But Middleby said it was encouraging Aga investors to continue to back its offer, with no firm bid on the table from Whirlpool.

It added: "Middleby encourages Aga shareholders to consider the motivations of the approach by Whirlpool, in particular why it has waited until this late stage in the process to approach Aga.

"Middleby urges Aga shareholders to be wary that there is no certainty Whirlpool will ultimately make any offer."

Aga has also said it still backed the 185p-a-share offer made by Middleby in July in the absence of any firm bid from Whirlpool.

The takeover battle comes after Aga appointed advisers Rothschild to help assess ''development opportunities'' for the business at the start of this year.

It first revealed talks with Middleby in June, before agreeing the deal a month later.

Middleby has sought to allay concerns over the future of Aga's UK manufacturing operations as part of the takeover.

Swedish Nobel Prize-winning physicist Gustaf Dalen built the first Aga in 1922 and manufacturing was under way in the Midlands seven years later.

The ovens are now built in Coalbrookdale, Shropshire.

Aga maker Allied Iron Founders was bought by Midlands-based engineering firm Glynwed in 1969. The Aga part of the business was split from the rest of the company in 2001.

Over the years, the oven acquired a place in British popular culture - featuring in The Archers as well as romance novels that became known as ''Aga sagas''.

Whirlpool has until 5pm on September 14 to make a firm offer for Aga or walk away.