Coach Wayne Bennett says Great Britain "underperformed" on their comeback tour, which ended as a 4-0 whitewash with defeat by Papua New Guinea.
The Kumuls fought back from 10-0 down to stun Great Britain 28-10 and pile pressure on the Lions and England head coach, who is now out of contract.
The loss followed two defeats by New Zealand and another against Tonga.
"We are disappointed," Bennett told BBC Sport. "We've underachieved, I don't think there's any doubt about that."
The tourists lost captain James Graham to a head injury inside the first minute after the prop was clattered by PNG's Luke Page in the game's first hit-up.
- 26 Oct: Tonga beat Lions 14-6 in tour opener
- 2 Nov: New Zealand win 12-8 in Auckland
- 9 Nov: New Zealand win 23-8 in Christchurch
- 16 Nov: Papua New Guinea win 28-10 in Port Moresby
Tries from makeshift winger Blake Austin and Josh Hodgson ensured the tourists led 10-6 at the break, although Edwin Ipape's response just before the interval proved decisive.
Bennett's side, who laboured in attack throughout the tour which took in New Zealand and PNG, failed to match their earlier defensive displays as the Kumuls ran in four unanswered second-half tries to earn just their second ever win over the Lions.
The Australian said PNG's win, just like Tonga's triumph over Great Britain to start the tour, was proof that the "international game has improved enormously".
The 69-year-old continued: "It is only getting better, and that is what we want.
"But at the same time, we have still underperformed, I'm not using that as an excuse. I'm just telling you how it really is.
"I don't want to make any excuses about our performance, we went out there and we were pretty good for 38 minutes and for the next 42 we were out of the game."