Michelle Ryan will guest star alongside David Tennant in this year's first Doctor Who special at Easter, prompting speculation that she may get a permanent role as the Doctor's companion when the show returns for a new series in 2010.

The former EasterEnders star will play the mysterious Lady Christina de Souza, who joins the Doctor on a bus trip which takes a "detour into danger", the BBC said. The special is scheduled for broadcast in April.

"I'm a huge fan of Doctor Who and very excited to be joining David Tennant and the Doctor Who team," said Ryan. "It is such a fantastic show and I can't wait to get started."

Ryan will be joined in the Planet of the Dead special at Easter by comedian Lee Evans, who plays a character called Malcolm, whose life becomes connected to the Doctor's under extraordinary circumstances.

Planet of the Dead is the first of four Doctor Who specials that will air in 2009. The shows will be the swansong for Tennant and Russell T Davies – the executive producer and showrunner responsible for the sci-fi drama's hugely successful revival in 2005 – who will both bow out after the Christmas 2009 special to make way for new Doctor, Matt Smith.

However, BBC executives have not yet announced who will be Smith's companion, increasing speculation that Ryan could get the role full time as the show has a habit of re-using actors and characters from earlier programmes.

Catherine Tate, for example, was originally cast in a Christmas 2006 special, The Runaway Bride, then returned to star alongside Tennant as the Doctor's companion in series four last year.

"Michelle is one of the most sought after young actors in the country and we are delighted to announce that she will be joining the team," said Davies.

"As always the script is being kept strictly under wraps however we can reveal that Lady Christina is a woman with a mysterious past who's going to have a huge impact on the Doctor."

Ryan has previously worked with Steven Moffat, who takes over from Davies, as he wrote the six-part BBC1 series, Jekyll, that she starred in in 2007.

She was also on-screen last year in BBC1 fantasy drama Merlin playing sorceress Nimueh, alongside veteran actor John Hurt, before which she starred in US broadcaster NBC's 2007 revival of Bionic Woman, which was axed after one series. The show was broadcast in the UK by ITV2.

Ryan also featured alongside former Doctor Who assistant Billie Piper in ITV1's 2007 adaptation of Jane Austen's novel Mansfield Park.

However, Ryan is best remembered for her breakthrough role in EastEnders playing Zoe Slater in the BBC1 soap between 2000 and 2005.

Filming of Planet of the Dead, written by Davies and Gareth Roberts, began this week in Wales.

The show will also see the return of Noma Dumezweni as captain Erisa Magambo, who previously appeared in an episode of last year's series called Turn Left.

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