Russell Slade began his managerial career at Notts County

Leyton Orient manager Russell Slade, target of managerless Cardiff City, has tendered his resignation as O's boss.

On Monday, Orient rejected an approach by the Bluebirds for 53-year-old Slade, who had been their manager since 2010.

Orient confirmed on Wednesday they had received a letter of resignation.

"This has not happened by chance after the club rejected this afternoon, a proposal from the general manager of Cardiff City regarding the transfer of Russell Slade," an O's statement read. external-link

"The lawyers of the club are now studying the file."

BBC Wales football correspondent Rob Phillips "Slade is the man that owner Vincent Tan in particular has wanted for Cardiff City, and there were two options. Either the clubs reached a deal on a compensation package or he would have to resign and then take the job at Cardiff. Well, he's done the latter, and I confidently expect him to be manager - or head coach - of Cardiff City in the next couple of days."

If his resignation is accepted, it would open the way for him to hold talks with Cardiff about a move to the Championship club.

Slade was in charge for Orient's Capital One Cup defeat by fellow League One side Sheffield United on Tuesday, and after that game, spoke of his "frustration" at not being permitted by his current employers to talk to Cardiff.

"It's been turned down and that's all I can say, and whether Cardiff come back again I'm not sure," Slade said.

"Maybe they'll move on. It's a big club and it will be a big job for somebody that eventually ends up with [it]."

The Welsh side have been without a manager since Ole Gunnar Solskjaer stepped down last week.

Slade - named League One manager of the year for the 2013-14 season - has been one of the favourites to take over since Solskjaer departed last Thursday.

Scott Young and Danny Gabbidon, who have been put in caretaker charge, oversaw Saturday's 2-2 draw at Derby and Tuesday's League Cup defeat by Bournemouth.

Former Leyton Orient manager Martin Ling "For Russell it's a big step and a chance to manage at Championship level and you know that chance may never come along again. I don't blame him for taking that option if that's the option he's taken. It's obviously an awkward situation, but only two games ago the Orient chairman was saying he had one game to save his job so you can't have it both ways"

Slade, who held talks with Orient owner Francesco Becchetti about his position last week, had said he was "proud" to be linked with Cardiff, who were relegated from the Premier League at the end of last season.

The Bluebirds sacked Malky Mackay in December 2013, with successor Solskjaer departing after less than nine months in charge.

"On Friday morning, we were contacted by a lawyer close to Cardiff City and Russell Slade, who asked us to give the permission for Cardiff to talk to Russell," Orient's statement on Monday read. external-link

"On Friday evening, Russell asked the club for the authorisation to talk to Cardiff. On Saturday the general manager of Cardiff approached us in order to ask us the permission to talk to Russell.

"In all cases, we denied Cardiff the permission to talk to Russell and did not authorise Russell to talk to Cardiff."

Slade led the O's to third place in League One last season but saw his side beaten on penalties by Rotherham in the play-off final.

The former Yeovil and Brighton manager has since come under pressure from Becchetti following a slow start to the new campaign, with Orient 17th in the table.