Clayton Donaldson's departure has left a bitter taste in the mouth for many Brentford fans.

The Bees star striker quit Griffin Park on Wednesday to sign for Birmingham City on a free transfer after his contract in TW8 expired.

If joining a Championship rival wasn't bad enough, it was the manner of Donaldson's exit which was a real kick in the teeth for supporters.

Donaldson had been a great servant to the club, plundering 53 goals in 155 games since his switch from Crewe Alexandra in 2011.

Goal machine: Donaldson smashed 18 goals last season as Brentford won promotion

His contribution can't be underestimated and it's highly unlikely Brentford would have won promotion had it not been for his haul.

Donaldson was in negotiations over a new deal since before Christmas but his future was put on hold while the Bees tried to get out of League One.

However, the Bradford-born star always spoke of his desire to remain in west London, providing they secured a spot in the Championship.

That part of the bargain was achieved back in April and the club acted swiftly to table an improved two-year contract offer, with the option of a third year, to their top scorer.

However, it soon appeared clear that the Bees had a fight on their hands to try and keep Donaldson after Get West London revealed Bolton Wanderers, Birmingham and newly-promoted Rotherham United were among five other Championship clubs chasing his signature.

Brentford were still in pole position to wrap up a deal, but were keen not to be drawn into a bidding war.

However, rumours of a move to St Andrews started to gather pace while the forward jetted off to the US on his honeymoon.

The Bees were in the dark while talks between Donaldson's advisers and Birmingham reached an advanced stage last week, with a deal all but done.

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And on the day the rest of the Brentford squad reported back to Jersey Road for pre-season training, Donaldson's switch to Blues was confirmed on a two-year contract.

At 30, this is likely to be his last major deal, so it's difficult to begrudge him moving to a bigger club.

Birmingham were League Cup winners and an established Premier League outfit only a few years ago.

To make it worse for Bees fans, however, it is the third time since a rivalry of sorts developed between the clubs in the early to mid-1990s that the Midlanders have plundered Griffin Park to boost their strike force.

The rivalry reached it's peak in 1994-95, when they were going head to head for the one automatic promotion place from the third tier – the other taken away because the Premier League wanted to reduce its numbers.

FA Cup action: Donaldson in action with Chelsea's David Luiz

Birmingham emerged triumphant, then rubbed salt into the wounds by snatching Nicky Forster two years later, and DJ Campbell a decade after that. Both times, Brentford sides seemingly set for promotion blew it after losing their star striker.

At least this time promotion was in the bag before Brum struck, and the Bees will have the chance to prove Donaldson wrong next season.