In a deal worth $4.85 billion in cash up front, AT&T has agreed to lease around 9,100 of its wireless towers – and sell another 600 – to Crown Castle International.

The arrangement might see the US operator eventually bag over $9 billion.

As the leases expire – the average length is around 28 years – Crown Castle has the option to buy them for about $4.2 billion.

A tower deal, giving AT&T more financial leeway to spend on 4G and perhaps invest abroad, has been rumoured for some time.

“This deal is good for AT&T and our shareholders,” said Bill Hogg, SVP of network planning and engineering at AT&T. “This deal will let us monetise our towers while giving us the ability to add capacity as we need it. And we’ll get additional financial flexibility to continue to invest in our business, maintain a strong balance sheet and return value to our shareholders.”

AT&T will sublease capacity on the towers from Crown Castle for a minimum of 10 years for $1,900 per month per site, with annual rent increases of 2 per cent. AT&T has the option to renew up to a total of 50 years.

Due to report its Q3 results this week, AT&T said there would be no significant impact to its financial results from the transaction.

The US number two operator expects the deal to close by the end of 2013.