Mike Ashley says his money is "like wallpaper" and he cannot afford to fund a spending splurge at Newcastle United amid growing frustration from manager Rafael Benitez.

Last season's Championship winners have added four players to their squad ahead of the new season, as well as turning Christian Atsu's loan from Chelsea into a permanent deal.

However, Benitez has grown frustrated at the club's inability to land certain targets, prompting speculation that he could quit – with comparable rumours having followed a similarly stalled January transfer window.

But Ashley says the Spaniard will have to make do with a restricted budget, as the club are unable to generate vast sums of money to keep up with the spending power of the Premier League's elite.

"If you say to me I'm wealthy, okay, in theory I'm a, whatever, billionaire, maybe a multi-billionaire," Ashley told Sky Sports.

"But in reality, my wealth is in Sports Direct shares that are the same as wallpaper, I don't have that cash in the bank.

"So I don't have that ability to write a cheque for £200million, I don't have it, it's very simple, it's not there. I would have to sell the Sports Direct shares to fund that.

"People outside of football looking in, and the way sometimes it's portrayed, is that those sort of wealth terms are in the bank – they're not and I've got to make that crystal clear.

"I am nowhere near wealthy enough in football now to compete with the likes of Man City etcetera.

"Basically, it's a wealthy individual taking on the equivalent of countries, I cannot. I cannot and I will not."

Upon Newcastle's promotion in May, Ashley said Benitez could have "every last penny" of the Premier League riches that would come the club's way, with sums north of £100m in television deals bolstering the coffers of newly promoted rivals Brighton and Hove Albion and Huddersfeld Town to such an extent that both have broken their transfer record numerous times since promotion.

But, when asked how much money Benitez could expect, Ashley bluntly replied: "Not enough. Very simple, it's not enough and Rafa knows that.

"It's not enough, it's not a secret, every penny the club generates he can have, but it won't generate enough. It's Newcastle United: it doesn't have a £40million-a-year naming stadium rights deal, it doesn't. 

"So I don't want fans to watch this interview and think 'that's great, Rafa's getting £150million in the morning' – he's not.

"Rafa is, with [director] Lee Charnley's help... Lee Charnley answers to Rafa, not the other way around so were crystal clear Rafa makes all the final decisions on players out, players in, but he has to do it with the money the club has."