The bid, valuing the company at pounds 21m, drew a cautious response from Chiltern management. The company urged shareholders to take no action until the Board had a chance to review its options.
The timing of the offer surprised few analysts, coming on the heels of the Government's Green Paper on media cross-ownership, under which companies can now own up to 35 radio licences, rather than the previous maximum of 20.
GWR, with 20 licences, had made no secret of its intention to grow by acquisition, having purchased 12 new licences in the Midlands and Anglia since last year.
GWR has received an irrevocable agreement from Capital Radio to tender its 20 per cent stake, as well as a comfort letter from the Daily Mail and Emap for a further 29.9 per cent.
Chiltern, with nine independent radio stations, was the target of an attempted takeover last year by CLT UK, a subsidiary of the Luxembourg broadcaster. That offer was allowed to lapse after major shareholders declined to tender.
Other radio shares were up in sympathy yesterday, with Metro Radio cllmbing 10p to 478p and Capital by 7p to 466. Analysts predicted further shuffling in the wake of more liberal ownership rules.
If the offer is successful, GWR intends to bring Chiltern's stations under its central management. "We have a lot of experience with running multi-site radio," Ralph Bernard GWR chief executive, said.
The radio business has been a lucrative one in recent years, with advertising revenues growing faster than those in other media for the second year running. Mr Bernard said he believed the remarkable growth was likely to continue.