Warren Gatland has explained that the Six Nations was the key benchmark for his Lions squad, which Rory Best was among those to miss out on this morning,

Ireland hooker Best effectively lost out to surprise inclusion Dylan Hartley. England captain Chris Robshaw and Scotland back-rower Kelly Brown were also among the biggest casualties of Tuesday's squad announcement.

Simon Zebo, Mike Ross, Donnacha Ryan and Ian Madigan were other Irish players thought to have a chance of inclusion who did not make the grade.

Danny Care, Brad Barritt, and Joe Launchbury of England and Scottish trio Greig Laidlaw, Ryan Grant and Euan Murray also failed to make the cut. Several players based in France were also left out, with James Hook and Jonny Wilkinson among those overlooked.

Best got off to a roaring start in the Six Nations but as Ireland's form dipped, with the lineout in particular suffering, his chances appear to have receded.

Gatland said: "We pretty much finalised the squad last night - the management assembled yesterday (Monday) afternoon. There was some robust debate and I know there will be some disappointed players. There were some really healthy discussions in every position.

"We've looked at performances over the last few weeks and players who have come back from injury, but the benchmark was the Six Nations.

"Ideally you'd like to pick everyone but we've had a thorough process and we feel like we have an incredibly talented squad with a lot of depth in a lot of positions."

Gatland's squad is eight players smaller than the bloated 45-man squad which travelled to New Zealand in 2007, and every man should all be aiming for Test selection.

He added: "All 37 of these players should go to Hong Kong and Australia with the ambition of making the Test side."

Speaking after the official squad announcement, Sam Warburton described his appointment as British and Irish Lions captain for the 2013 tour to Australia as the "hardest secret I've ever had to keep".

The 24-year-old openside was chosen for the post ahead of Ireland's Brian O'Driscoll and Paul O'Connell by head coach Warren Gatland, with whom he has forged a close relationship with Wales.

Bookmakers suspended betting on Warburton's appointment two weeks ago and the Grand Slam-winning skipper spoke of his difficulty in keeping the news to himself.

"Warren told me a week ago Sunday. For the last nine or 10 days it's been the hardest secret I've ever had to keep," he said at today's squad announcement press conference in London.

"You just want to tell people, but I managed to keep it to myself.

"It's unbelievable - an accolade that very few people are able to achieve. To have the opportunity to do this is an unbelievable honour.

"Obviously it's going to be new for me and I'm going to learn along the way. I can't sit here and say I know what's going to happen because I don't.

"There's a good Welsh contingent of leaders who I can rely on and there are other experienced players in the squad who can help me along the way.

"It's an experience I really can't wait to begin. I'm really looking forward to it now and will just keep doing what I do for Wales because I guess that's what Warren has picked me for."

Gatland explained his thinking behind choosing Warburton, who heads a 15-strong Welsh contingent with 10 English, nine Irish and three Scots completing the touring party.

"In the last two or three years he's been the most successful captain in the northern hemisphere," Gatland said.

"When we sat down to pick a squad - and it's a fantastically strong squad - we decided that Sam would be one of the names among the starting Test XV.

"I have a huge amount of respect for Sam. He's an absolute professional who leads from the front and I believe he'll do a fantastic job."

By his own admission Warburton may lack experience, but there was no shortage of ambition from the Cardiff Blue as he targeted an unbeaten tour and first Test series triumph since 1997.

"I'm always the eternal optimistic anyway, so I'll go out there with the intention of winning every match," he said.

"I was chatting to (tour manager) Andy (Irvine) last night and he said that it's not something that's been done before, so that will be a great challenge to set out to the players.

"Everyone who's been picked is naturally a great competitor and will be desperate to give everything to the Lions.

"Nobody on the tour has had a Test series win and I'm sure that's something that we'd all like to achieve."