The 38 staff of the Commercial Road branch of NatWest were found to have provided uniquely outstanding customer service, with a '100 per cent quality service satisfaction' rate.
A mystery shopper and customer poll put the branch top of the 1,628 branches of NatWest in England, Scotland and Wales, with it being the only one to achieve the 100 per cent rating.
As well as customer feedback forms, NatWest staff also visited incognito to score the branch on issues ranging from staff appearance and friendliness to the layout of the office and the length of time customers were kept waiting.
Sarah Sidney, manager of NatWest's Portsmouth Commercial Road branch, said the accolade reflected well on the whole team.
She said: 'When you work so hard as a team to deliver a quality service, it's great to receive a national award from the bank. It means so much to us to be in the top spot, and we are thrilled to be recognised as offering this level of support to our customers.'
But perhaps the stand-out star was 18-year-old customer advisor Jake Alford. The survey singled him out for special praise, and found him 'very helpful, friendly, polite and knowledgeable'.
The satisfied customer noted: 'He promised to give me a call to follow up my visit, and did so at the exact time and date that we had agreed. Basically, everything he did was exactly right.'
Mr Alford said: 'I feel very proud to have received this feedback, but at the same time it's what I expect of myself.
'It's my job to provide excellent service to every customer who walks into our branch, and I'd be disappointed if I didn't deliver that every time. The reaction from my colleagues has been great, though, they are delighted for me and it's fantastic to be part of a winning team.'