BT's telly heart-throb Kris Marshall has been hooking up with a blonde neighbour behind his girlfriend's back- and his secret lover is PREGNANT, we can reveal.

MUM-TO-BE: Neighbour

Now the actor is sweating on a DNA test to see if he is the baby's father.

And his loyal girlfriend Kelly Eastwood has no idea that Kris-known to millions as the BT boyfriend-has been pushing another woman's buttons.

The lanky star is understood to have been seeing the 30-year-old beauty-who lives a five-minute walk from his home-for several months until she fell pregnant around four months ago.

A pal of the neighbour said: "With the pair of them spending so much time together and sleeping together, she fell in love.

"The affair carried on until sometime around Valentine's Day, when she told Kris she was pregnant.

"At first he said he didn't want anything to do with it and hasn't seen her since-but he must have changed his stance as he now wants her to have the DNA test.

"She's obviously devastated. She claims she's had no other lovers and is adamant it's his. She thought he cared for her, but now has to have a test to prove what she says is true."

While Kris, 35, was secretly bedding the blonde he was out and about with Kelly, 27-his girlfriend of two years- at premieres and other celebrity events in London.

"Kelly has her own place and was suspicious-but she had no idea what was really going on," said the neighbour's pal. Kris has told friends he will accept his responsibilities if a DNA test shows he is the father.

But the star-who shot to fame in comedy My Family-faces an agonising five-month wait to find out because his lover has refused to allow a test until the baby is born.

A source told us: "She is convinced he is the father, but Kris maintains it is unlikely he is. He accepts there is a chance he may be the dad and as a result wants a DNA test to be sure."

COMMERCIAL SUCCESS: Kris as Adam in BT ad

A spokesman for Kris said: "If it turns out positive he will hold up his hands and accept his parental responsibilities."

The lanky actor is away filming in the Caribbean. A spokesman said he would be staying in Puerto Rico filming his latest movie for another month.

When asked if there would be a showdown between Kris and Kelly-who met on the set of movie Death At A Funeral in 2007-his spokesman said things were being "handled".

It is believed Kris got to know the neighbour through friends where they both live in Windsor, Berks. She got close to him last summer and began visiting him at his £400,000 home near the centre of the town.

The pair started sleeping together after a series of late- night drinking sessions.

Meanwhile he continued his public relationship with Kelly, who lives in Fulham, south west London.

Kris first shot to fame as the gormless elder son in the prime time BBC sitcom My Family, alongside Robert Lindsay and Zoe Wanamaker.

He was in 44 episodes of the BBC comedy, watched by 12 million fans at its height. In 2003 he appeared in the worldwide smash hit comedy Love Actually. Kris a loser in love who heads to America in the hope that Stateside women will fall for him because of his "cute British accent."

But his role in the adverts for BT has brought him a regular audience gripped by the ups and downs of the relationship between Kris's character Adam and older divorcee and mum Jane, played by Esther Hall.

The star had a brush with death last year when he was hit by a car during a friend's stag night and ended up in a coma for 48 hours.

Kris recently said of his relationship with Kelly that two years was "a long time, for me. Perhaps I'm not good husband material".

When the News of the World contacted the pregnant neighbour last night she said: "I'd like to know who told you this." She made no further comment.

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