A royally good cause! Prince Harry's girlfriend Meghan Markle joins stars including Lady Gaga and Charlize Theron in a call to provide education for poverty-stricken girls

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Their relationship appears to be going from strength to strength and it certainly seems that Prince Harry and his girlfriend Meghan Markle have one major trait in common - a passion for activism.

Suits actress Meghan has joined a host of celebrities in putting her name to an open letter demanding an education for the millions of girls suffering from poverty.

The open letter - which has also been signed by Facebook COO Sheryl Sandberg and YouTube CEO Susan Wojcicki, actors Charlize Theron and Blake Lively, musicians Bob Geldof and Lady Gaga, among others ­– calls on world leaders to take urgent action and empower girls through the classroom.

Meghan, pictured in March last year, has signed the ONE Campaign's open letter

Stars including Charlize Theron, left, and Lady Gaga, right, are also supporting the Poverty Is Sexist Initiative

The open letter signed by Meghan was organized by the ONE Campaign as part of its Poverty is Sexist initiative, and says in part: 'We are coming together and uniting across our divides to get every girl into school and to make sure she gets a quality education once she's there.' 

This is not the first time Meghan, 35, has used her high profile to support causes close to her heart, as she's an ambassador for Canadian charity World Vision and is also a United Nations women’s advocate for Political Participation and Leadership.

Harry, 32, is well known for his work with a number of charities across the globe - in particular founding Sentabale, an organisation which provides support for children living with HIV in Southern Africa, and he is also a founding patron of the Invictus Games.

This is not the first time Meghan, 35, has used her high profile to support causes close to her heart

The pair, who are believed to have been dating since early last year, recently enjoyed their first holiday as a couple.

Having spent Christmas apart, Meghan and Harry celebrated the new year together before flying out to Tromso, on the Norwegian fjords, where they also whale watched and enjoyed romantic lake trips.

And the pièce de résistance of the loved-up couple's first trip away together was looking up into the night's sky at the Aurora Borealis.

A source told The Sun: 'Harry wanted to make this first holiday something Meg will never forget. 

Harry, 32, is well known for his work with a number of charities across the globe

He'd put a lot of thought into it and wanted to make it as romantic and special as possible.'

The couple opted for a secluded cabin where they could enjoy maximum privacy.

Their accommodation was reportedly picked following advice from Harry's friend Inge Solheim, a polar explorer who has previous organised trips for the royal, such as a skiing holiday in Kazakhstan.

Kensington Palace has declined to comment, and a spokeswoman would neither confirm or deny the trip had happened.