Lockwood was in last season's PFA League One team of the year

The left-back, 30, has signed a two-year contract at the City Ground.

Lockwood had only signed a new two-year deal at Orient, with the option of a further year, at the beginning of May.

"He has chosen to exercise the clause he had in his contract, allowing him to leave for a certain fee," O's boss Martin Ling told his club's website.

I think he felt this was too good an opportunity to turn down Leyton Orient boss Martin Ling

"He's been very close to moving on in the past, and I think he felt this was too good an opportunity to turn down.

"He's been a great servant to Orient and it's probably his last chance to move to a big club, so I wish him all the best."

He has scored 50 goals in 328 league appearances for Orient, since joining from Bristol Rovers in 1998 - and found the net 11 times last season.

Lockwood helped Orient win promotion from League Two in 2006.

He was also selected in the Professional Footballers Association's "Team of the Year" for League One by his fellow professionals last season.