Careless Portsmouth slipped to a 30-29 defeat at Old Tiffinians in London three south west.
It was another damaging blow to the city club's promotion hopes.
Head coach Neil McRoberts insists his side had only themselves to blame.
He said: ‘We got ourselves into a good winning position with 15 minutes to go then let it slip.
‘All we had to do was see the game out because the game was in our grasp.
‘Instead we made the mistake of thinking that the job had been done.
‘We lost our concentration and it is a harsh lesson to learn.
‘Old Tiffinians got themselves in front 30 seconds from the end and there was no coming back.’
Portsmouth scored two first half tries to lead 15-9 at the break.
Another two tries after the interval saw Portsmouth leading 29-16 with fifteen minutes left to play.
‘In reality we stopped doing the hard work,’ said McRoberts.
‘That is a mental thing the players need to remedy.
‘You have to keep playing to the final whistle.
‘On the positive side at least we did manage to pick up two bonus points.’
United Services Portsmouth drew level on points with their third-placed local rivals courtesy of a 22-21 home win against Warlingham.
Bottom club Petersfield found leaders Winchester too powerful in their 69-0 away defeat.
In Hampshire two Southsea Nomads went down to a 29-3 defeat at Chineham.
The points for the visitors came from a first-half Jonathan Branston penalty.
Nomads captain, Ben Horrod said: ‘We knew this was going to be a tough match and one that we needed to play well in order to get the result we wanted.
‘But we had a poor start and the first half was one of our poorest of the season so far.
‘In the second half we started to play our rugby but Chineham defended well and we were unable to convert the opportunities that we created.
‘We will take a lot from this loss and make sure that this is the last time that we have a performance like this this season.’