Chelsea are preparing an improved bid for Everton striker Romelu Lukaku after having a £57m ($76m) offer for the Belgian international rejected over the weekend.

The Daily Mirror says the Blues are ready to pay up to £68m to bring the 23-year-old back to Stamford Bridge, after selling him for £28m to the Toffees two seasons ago.

However, Everton are reported to be unwilling to sell the player and are holding out for a fee of £75m.

The biggest ever transfer fee paid by a British club is the £59.7m Manchester United shelled out to Real Madrid for Angel Di Maria in 2014.

Talks between Chelsea and Everton over Lukaku have been going on for two weeks, with the Belgian striker understood to be in favour of returning to Stamford Bridge.

But the Toffees are unwilling to budge over the transfer fee given Lukaku's importance to the Merseyside club.

The 23-year-old scored 25 goals in all competitions for Everton last season and has three years left on his Goodison Park contract.

Costa future uncertain

The Mirror says Chelsea will offer a "lucrative" five-year contract to Lukaku if they can negotiate a transfer fee with Everton.

The Blues turned their attention to the Belgian forward after Real Madrid made it clear that Alvaro Morata would stay at the Santiago Bernabeu.

Romelu Lukaku
Romelu Lukaku, 23, was sold by Chelsea to Everton for £28m in 2014Getty Images

Chelsea's first-choice forward Diego Costa missed the pre-season friendly against Real Madrid through injury and has been linked with a summer return to Atletico Madrid.

"I can say that today Costa is a Chelsea player," Blues boss Antonio Conte told the club's official website after the 3-2 defeat against Real in the United States.

"He didn't play in these games because of injury and if he solves the injury and I see in training he's in good shape, it can be possible to see him in the next game against Milan.

"But I can tell only this. Today Costa is Chelsea's player. Tomorrow if you ask me if Costa will remain with us, I don't know."