The pair were encouraged to get together by their mutual pal Keira Knightley after the trio went out for dinner in May - and the couple have been trying to keep their blossoming romance under wraps ever since, reports The Sun.

The 32-year-old Welsh actor, who is best known for playing the role of gay lawyer Kevin Walker in family drama Brothers And Sisters, is Sienna’s first serious romance since her split with Jude Law in 2006.

Despite rumours that she is dating P Diddy, pals say she only has eyes for Matthew.

A friend said: "This is the first bloke Sienna has been serious with since she finally split with Jude.

"These vicious allegations of her being with Diddy and splitting up his relationship with Kim Porter are awful. But they have actually worked in her and Matthew’s favour a bit.

"It has taken the heat off their relationship because people have been looking for a spark between her and Diddy.

"Sienna and Diddy are party pals.

"They love to go out and have a good time but that’s where it ends, they have had some wild nights in Los Angeles, London and St Tropez but he isn’t someone she is going to take home to her mum."

Sisters in fashion - July 30 2007
They’re not the first siblings to create a fashion line, and they won’t be the last, but Sienna and Savannah Miller were in New York to mark the launch of the spring-summer range of their Twenty8Twelve by S. Miller collection.

And while fashionistas shouldn’t expect to see it on a runway soon, Sienna already has high hopes for the range.

"I would love to have a show someday, maybe if we are bought by the Gucci Group. Then we can have one," she told Women’s Wear Daily.

Sister Savannah, who used to design for Alexander McQueen, added: "For me, there has to be a real reason to have a show. At McQueen, the clothes called for a production and he really puts on a real show."

During their short visit to the city, Sienna and Savannah managed to fit in some quality sister time.

"We had a really great sister moment today and went to a little place down the street to get our nails done," Sienna said.

’Sister act’ for Sienna - July 13 2007
Sienna Miller is apparently launching a fashion label with her older sister.

Savannah, who has recently created a capsule collection for Shanghai Tang, defended setting up a line with her equally stylish sister.

The 28-year-old designer told the Daily Express: "Of course people will say I’m jumping on Sienna’s bandwagon and cashing in on her fame but this is something we have talked about for years."

Sienna is also excited about the range, called Twenty8Twelve, that will launch in September. The line has been named after the actress’ birthday.

"Sav has been the closest person to me my whole life," Sienna smiled. "Now I’m working with her - what could be more fun?"

She previously said she plans to be very hands-on with the project.

"This isn’t about just lending a name to something and not really being involved," she said.

Savannah is a graduate of Central Saint Martins and has also previously lent her talents to Alexander McQueen, Betty Jackson and Matthew Williamson.

Sienna backs worldwide shape-up - June 4 2007
If Sienna Miller’s new workout catches on, it could help save more than our waistlines - the entire planet, in fact.

She’s backing a new programme that helps you shed a different kind of pound, the carbon kind.

"My CO2" is billed as a way for individuals to "shed tonnes" by reducing their carbon emissions.