Marks & Spencer is preparing a major push to sell more high street brands alongside its own clothes as it attempts to lure shoppers away from rivals Next and John Lewis.

M&S is understood to be planning to increase the number of third-party brands it sells to 100 from 60, in a move it expects will significantly increase sales.

It has already started selling clothes from brands including Ted Baker, Superdry and Crew Clothing, alongside its own fashion lines.

Senior figures believe third-party sales could eventually be as high as £1bn – almost a tenth of total revenues for M&S, which last year posted sales of £10.9bn.

The private ambitions mark a significant step-up from plans laid out publicly by the retailer, which has said it is targeting £400m of sales from third-party clothing brands in the medium-term.

It would be an even bigger increase from current figures, with sales of non-M&S brands accounting for just under £30m of its revenues over the three-months to the end of December – and around 8pc of the total online sales.

The push into third-party brands is being driven by Nishi Mahajan, who the company poached last month from her role running Amazon's fashion business.

Part of the growth will be driven by clothing, but M&S is also seeking to sell beauty sportswear and homeware brands on its website.

The step-up will mean more investment from M&S into warehouse space, although it is not currently seeking to buy any specific depots.

M&S first started to sell brands on its website two years ago, and has enjoyed major growth from shoppers looking for high street names on its site.

In the three months to the end of December, sales of third party brands were up 50pc.

Almost all shoppers buying a non-M&S brand online also bought something own-brand.

The push by M&S comes after a similar rollout of more lines on rival Next, which has gone from 500 third-party brands to 1,000 in around three years.

A spokesman for M&S said the company was, however, taking a different approach to Next, and instead was being “very clear that curated choice is key – not a plethora of brands that are hard for our customers to navigate”.

Richard Hyman, a retail analyst at Thought Provoking Consulting, said the change would be a delicate balance for M&S.

He said: “A big question for me is to what extent are sales of M&S products being cannibalised by sales of third party brands?”

Clive Black, of Shore Capital, said the third-party brand sales would be lower-margin for M&S, but added that adding more products made sense as it would mean bringing shoppers onto M&S's website or into stores to collect, where they may be tempted to buy other things, such as their groceries.

The move by M&S comes after a survey suggested it was on course to overtake the John Lewis Partnership in the rankings of Britain's biggest retailers as soon as the end of this year.