LONDON (Reuters) - U.S. financial software company SS&C Technologies has been given until May 4 to gatecrash a 1.5 billion-pound takeover of Fidessa or walk away from the British firm, the UK's Takeover Panel said on Monday.

Fidessa, a provider of trading systems to financial companies, said on April 5 that it had received bid approaches from both SS&C and Ion. It then agreed to a takeover by Ion on April 20, ditching a Feb. 21 deal to sell itself to Swiss banking software firm Temenos in the process.

SS&C now has until 5pm (1600 GMT) on May 4 to make firm offer for Fidessa, the Panel said. Temenos has already ruled itself out of making a new offer for the London-listed business.

(Reporting by Ben Martin, editing by Louies Heavens)

By Ben Martin