A letter which launched the career of Isambard Kingdom Brunel and set the young apprentice on his way to become Britain's greatest ever engineer is expected to fetch £1,500 at auction.

The two-page letter of recommendation was sent in February 1830 from the second Earl Spencer to Davies Gilbert, a top engineer, recommending him to look at Brunel's designs for the Clifton Suspension Bridge

The following year, in 1831, Gilbert chose Brunel's design for the bridge.

Earl Spencer – an ancestor of Princess Diana – urged the politician to look at Brunel's ''beautiful" and "stupendous" designs and his "capacity as a civil engineer".

The letter was written when Brunel was aged just 23 and was serving under his father Marc on the building of the Thames Tunnel.

He had submitted several plans for the design of the Clifton Suspension Bridge in Bristol, which had all been rejected by engineer Thomas Telford.

But Earl Spencer had seen Brunel's potential and was so impressed by his abilities he wrote to Gilbert – who was on a bridge committee alongside Telford.

In the letter, Lord Spencer asks Gilbert whether: "Mr Brunel junior, the son of the ingenious Mr Marc Brunel, could call on him and show him his designs for Clifton Suspension Bridge."

He added: ''The drawings... are very well worth seeing merely as beautiful drawings, but besides that they exhibit a specimen of young Brunel's capacity as a civil engineer...

''He is ambitious of being backed by so high an authority as yours to the practicality and security of his plan for what really appears to be stupendous work of its kind, in the undertaking of which I understand that many of the principle and leading men of Bristol and its neighbourhood are greatly interested."

Later the same year Gilbert and the committee chose Brunel's designs for the crossing over the River Avon and building work began a year later in 1831.

Having his design chosen set Brunel on his way to becoming the greatest British engineer ever, creating famous works such as the Great West Railway in 1833.

Brunel had originally submitted four designs for the bridge to a committee headed by Telford, who rejected them all and proposed his own design.

But fierce opposition to Telford's plan forced the committee to hold a fresh competition – won by Brunel who was backed by Gilbert.

Brunel did not live long enough to see the bridge completed in 1864, as he died on September 15, 1859, aged 53.

Lord Spencer was a brilliant spotter of young talent and while First Lord Of The Admiralty, between 1794 and 1801, he gave Horatio Nelson his first independent command.

The letter is going on sale at Bonhams in London on March 23 and is expected to make £15,000.

Bonhams' books expert David Park said: ''A letter by this highly-regarded engineer will attract interest from collectors and institutions alike."