LONDON (REUTERS) - The surge in transfer spending could make the job of a Premier League manager increasingly difficult as it does not necessarily lead to an equivalent improvement in the squad, former England boss Sam Allardyce has said.

According to a recent report, English top-flight clubs have already spent over £850 million (S$1.51 billion) in the current transfer window and will likely top last year's £1.165 billion before the deadline on Aug 31.

"The difficulty for managers is the amount of money that is being spent," Allardyce told the BBC.

"The actual increase in the team is probably less than 5 per cent and that's what you're searching for now.

"You're searching for just a small percentage and having to pay millions and in some cases hundreds of millions for that little difference."

Allardyce, who stepped down as England manager after only one match in charge last September, said he could be tempted back into managing a national side.

The former Sunderland and Newcastle United boss had previously ruled out a return to football management at the end of his five-month stint at Crystal Palace.

"International football is completely different from Premier League football," he told Sky Sports.

"Manchester United, Chelsea and Arsenal aren't going to come in for me so maybe international football would be a challenge I would be willing to take.

"It would depend on the opportunity and how successful that international team can be."

In almost 20 years of managing in England's top division, Allardyce never once experienced relegation.