Were WillScot to succeed with its bid, the combination would create a modular space and portable storage powerhouse, one year after WillScot acquired Modular Space Holdings Inc, another office trailer company, for about $1.2 billion. That deal left WillScot with a $1.7 billion debt pile, almost as much as its $1.8 billion market capitalization.

WillScot has made an offer that values Mobile Mini in the mid-to-high $30-per-share range, and would be funded for the most part by using WillScot shares as currency, the sources said.

Mobile Mini shares jumped as much as 5.2% on the news, and ended trading up 3.4% at $34.75 on Wednesday, giving the company a market capitalization of $1.6 billion.

Negotiations between the two companies are ongoing and there is no certainty a deal will be agreed, the sources said.

Mobile Mini is also considering alternative transactions, including acquiring the U.S. assets of mobile storage company General Finance Corp, should the latter divest its Australian portable buildings and storage container business Royal Wolf Holdings, one of the sources added.

The sources asked not to be identified because the matter is confidential. Mobile Mini declined to comment, while representatives for WillScot and General Finance did not immediately respond to requests for comment.

Headquartered in Baltimore, Maryland, WillScot has a fleet of approximately 160,000 modular space and portable storage units. Private equity firm TDR Capital LLP is a major investor in WillScot.

Mobile Mini, based in Phoenix, Arizona, has a rental fleet of about 198,000 storage containers and office units, as well 12,700 liquid and solid containment units and pumps that it also rents out.

The Donerail Group, an investment firm led by former activist hedge fund Starboard Value LP executive William Wyatt, has built a stake in Mobile Mini to push it to explore a sale, Reuters reported earlier this month.

Beyond Royal Wolf, General Finance owns Lone Star Tank Rental Inc and Pac-Van, providers of portable storage, office and liquid storage tank containers, mobile offices and modular buildings. It is based in Pasadena, California, and has a market capitalization of $280 million.

(Reporting by Greg Roumeliotis in New York; Editing by Tom Brown)

By Greg Roumeliotis