Commenting on the call-up of Rory Best, Lions tour manager Andy Irvine said: "The British & Irish Lions leave on tour tomorrow (Monday).
"With uncertainty over the availability of Dylan Hartley to tour, and with the first of 10 matches only days away we had no option but to call up the additional player.
"Despite the circumstances we should congratulate Rory, who will join the squad tomorrow and fly with us to Hong Kong.
"A final decision on whether Dylan Hartley will take any part in the tour will depend on the outcome of any potential appeal."
Best played for Ulster in Saturday's RaboDirect PRO12 final against Leinster at the RDS. He was due to captain Ireland for their summer tour to North America, but will now link up with the Lions instead ahead of their departure for Hong Kong.
Speaking to the Ulster Rugby website, he said it was 'a massive honour' to be selected to tour with the Lions, admitting that it has not fully sunk in yet.
"It is hard to describe because it is the ultimate for a rugby player. To finally be selected makes playing professional rugby worthwhile," he said.
"All the lows that you get and the knocks and bumps, it more than makes up for them and for a few weeks ago when I thought that the chance had gone."
This is the 30-year-old's first Lions tour. He has played 67 times for Ireland and is the tenth Irish player included in Warren Gatland's touring party.
"The league final was bitterly disappointing from a team point of view. Today, from a personal view, it has lifted me out of a bit of despair after the game and it was great to get the news.
"I definitely want to make the Lions Test team. There will be 37 players getting on a plane to Hong Kong tomorrow aiming to make the Test squad and I am no different.
"I am as ambitious as everyone else. To be on the tour is unbelievable, but you have to go there and play your best rugby, put your hand up and see where that takes you," added the Banbridge clubman.