Canadian National Railway has asked ex-CEO Hunter Harrison to reconsider his expressed interest in taking over as chief executive at rival Canadian Pacific.

“CN has also put William A. Ackman of Pershing Square Capital Management, L.P., on notice to cease any efforts to induce breaches of Mr. Harrison’s contractual obligations to CN,” the company said in a statement on Friday.

CN said that on his retirement from CN at the end of 2009, Harrison agreed to a range of confidentiality, non-compete and non-solicitation provisions.

Ackman is preparing for a proxy battle to replace CP’s current chief executive Fred Green with Harrison.

© Thomson Reuters 2012