* Chilton to be fourth Briton on F1 grid in 2013
* Announcement leaves two seats available (Adds details and byline)
By Alan Baldwin
LONDON, Dec 18 (Reuters) - Max Chilton will be Britain's fourth man on the Formula One starting grid next season after back-markers Marussia said on Tuesday the 21-year-old would make his race debut with them in Australia in March.
Chilton, the reserve driver this year, will partner Germany's Timo Glock at the Russian-licensed former Virgin Racing team who have yet to score a point in the three seasons since they entered the sport.
"It's hard to put into words how I'm feeling today, with the announcement that I will be racing for the Marussia F1 Team in 2013," said Chilton in a statement.
"I am very fortunate to have spent the last six races with the Marussia F1 Team as reserve driver, which means that instead of a standing start I am already up to speed and at ease with the people, the culture, the systems and of course, the 2012 package."
Chilton will find himself lining up against compatriots Lewis Hamilton (Mercedes) and Jenson Button (McLaren), the 2008 and 2009 champions respectively, as well as Force India's Paul Di Resta.
The latest in a long list of British Formula One drivers took part in his first grand prix practice session in Abu Dhabi last month and replaces Frenchman Charles Pic, who has moved to Caterham.
While Chilton's talents behind the wheel have been praised by the team, his ability to bring much-needed funding with him was also critical to landing the drive.
Grahame Chilton, chairman of global reinsurance broker Aon Benfield and vice-chairman of Aon Group, has invested heavily in his son's rise through the junior series.
Chilton senior's Capsicum Motorsport purchased Carlin Motorsport in 2009, competing in the GP2 support series as Marussia Carlin Motorsport with Max Chilton as one of their drivers this year.
The Briton achieved two pole positions and two race wins in GP2 last season.
"We have spent a significant amount of time evaluating his performances during 2012, both in our own car in a testing situation and also from the pitwall as we monitored his progress alongside the other members of our junior talent pool during Grand Prix support races," said Marussia principal John Booth.
"He has shown himself to be extremely capable in the car."
Chilton's confirmation leaves two places remaining to be filled on the 2013 grid - one at Force India and the other at Caterham.
The 2013 entry list does not include Spanish-based HRT, who failed to pay the $500,000 entry fee by the deadline after seeking a buyer for the team.
Germany's Adrian Sutil is a frontrunner to return to Force India, the team he left at the end of 2011, although the team appear to be in no hurry to make a final decision.
Russian Vitaly Petrov is well placed to retain his seat at Caterham, having ensured the team finished 10th overall this year with his 11th place in Brazil, although there are other strong candidates for the seat. (Editing by Clare Fallon)