Co Antrim bus manufacturer Wrights Group has pulled up in a league table of the UK's most profitable firms for the first time.

The Ballymena company, which built London's Routemaster buses, joins medical diagnostic company Randox, based in Crumlin, to represent Northern Ireland in the Sunday Times BDO Profit Track 100 league table.

The firms employ 2,600 staff and have joint profits of £30m.

It's the second year in a row that Randox has secured a place on the list, which charts the private companies in Britain with the fastest-growing profits over three years.

The Sunday Times credited Wrights Group with "pioneering low-floor buses in Britain".

It has built 1,000 hybrid electric Routemasters for Transport for London - the so-called Boris Bus. In February, flamboyant London Mayor Boris Johnson confirmed a fresh deal for 200 buses on a visit to the Wrights factory - bringing the total number to 1,000.

The Sunday Times added that a "growing overseas business in markets such as India - where it opened a factory last year - helped it grow profits by an average of 94% a year to an annualised £13m in 2014".

And it said that Randox was now selling its medical testing kits to 145 countries, so that they are now used by 370 million people around the world.

According to the Profit Track, profits grew by an average of 68% a year to £17m in 2014.

Both companies are still strongly identified with their founders - Randox's managing director Dr Peter FitzGerald, and Wrights Group's William Wright, who founded the company with his father Robert in 1946.

Randox recently announced that it will be the main sponsor of the Grand National, beginning next year.

BI Electrical Services in Mallusk and Tobermore Concrete Products in Magherafelt were on the list last year, but do not make a 2016 appearance. Other UK companies on the list include domestic appliance manufacturer Gtech, craft beer maker BrewDog, sushi bar chain Wasabi and toy retailer The Entertainer.

And at 29, more than one quarter of the firms UK-wide are involved in construction and housebuilding - an increase of 14 on the year before.

Peter Burnside, managing partner in Northern Ireland at BDO, the title sponsor of the league table, said: "Medium-sized businesses across the UK are thriving, creating one in four jobs and responsible for one third of the country's total revenue.

"They are seizing the opportunities a recovering economy presents and the most profitable have remained focused on sustainable growth through investment in innovation, diversification and overseas expansion."