Pret a Manger is adding dozens more vegetarian specials to its menus after the popularity of meat-free options and a craze for avocados helped the chain achieve record sales last year.

Sales at the sandwich chain rose 13.9% to £676.2m in the year to December, with receipts at branches open a year or more up 7.5%. Profits before interest, tax and other items increased 14.5% to £84.3m.

Sales of vegetarian options grew by double-digits and Pret highlighted that it sold 17,000 each week of a new beetroot, squash and feta salad, outstripping chicken and salmon options.

Avocados, in favour with health-conscious foodies, were the chain’s fastest-growing ingredient. Sales of Pret lines containing avocado rose by more than a quarter – 26% – with customers consuming 5m of them in salads and sandwiches last year.

Clive Schlee, Pret’s chief executive, said the chain was particularly well-placed to take advantage of the fruit’s popularity as it made all its products in or very near its branches every morning, instead of in large remote factories. “Others can’t use them as well as we can because they go brown,” he said.

The 30-year-old company was founded by the entrepreneur Julian Metcalfe, who went on to create the Itsu restaurant chain and Metcalfe’s Skinny popcorn. It will convert a branch in Soho, London, into a vegetarian-only outlet for six weeks and branches will sell two vegetarian special options each month over the summer.

Schlee said the plans for the no-meat outlet had spurred Pret’s chefs to come up with dozens more ideas, some of which could then be brought into the main chain. He said thousands of customers had told the company they were trying to eat less meat and almost 10,000 customers voted on social media on options for improving the chain’s vegetarian range.

“People are thinking carefully about what lands in their stomach when they are eating and they are looking at vegetarian food,” he said. “My theory is that we would all eat more vegetarian food if it was more readily available, more colourful and a better texture. For so long it has been the poor cousin and we are trying to bring it up to the rest of the family.”

Sales at Pret’s US operation, launched in 2001, rose 13.8% at established stores – the chain’s fastest growing market. New products and larger stores to suit American tastes helped sales in the country, where Pret has 65 branches.

The privately owned chain, which started as a single shop in Victoria in London, opened 36 new branches worldwide last year, taking the total to 399, and made more additions this year such as a shop in Dubai international airport.

Schlee said sales had also been helped by good customer service, fostered by taking care of staff.

“One of our core values is happy teams mean happy customers,” said Schlee. He added that Pret had raised wages to maintain the business’s differential above the “national living wage” for over-25s introduced by the government this month.

Staff earn £7.50 an hour in London and can earn a £1 an hour bonus if they meet customer service criteria checked by mystery shoppers. Workers who put in a full day’s shift also get two free meals.

Schlee said the package was helping Pret stand out from rivals such as Caffe Nero and Eat, both of which have recently been criticised for cutting back on staff benefits. “We are now finding it easier to recruit people,” he said.

Pret expects to open 10 more US stores this year, including a second outlet at Penn station. The chain’s newly opened first store at the busy New York hub has the highest sales a square foot of any Pret in the world.

The group also opened sites at Paris-Gare de Lyon as well as Nice airport. Up to six more outlets are to open in France and three more in Hong Kong.

The majority of Pret’s 303 UK stores are in London. The company expects to open about 30 stores in the UK this year, up from 23 last year, 12 of which were outside London.

There are also plans to begin serving evening food, including alcohol, at two or three more outlets next month after trials at one central London store last year.

Pret figures at a glance

5m Number of avocados used by the chain last year.

26% Rise in sales of products containing avocado.

1m Number of free coffees given away to customers last year.

73,000 The number of chicken caesar & bacon baguettes –Pret’s most popular sandwich – sold a week in the UK.

1.5m Number of coffees its sells in a week worldwide.

401 Number of shops it has now.