LONDON (Reuters) - James and Lachlan Murdoch urged the British government to let their company buy pay-TV group Sky SKYB.L, saying on Wednesday that further delays to the $15 billion deal could sour the climate for foreign investment in Britain after Brexit.

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The Murdochs' Twenty-First Century Fox FOXA.O was dealt a blow last month when Britain's media secretary, Karen Bradley, said she was persuaded that buying Sky could give the family too much influence over the media.

Bradley said she was minded to refer the deal for a lengthy investigation, but has not yet announced her final decision.

“While we await the outcome of the regulatory process, important investment decisions will inevitably need to be deferred,” the two sons of Rupert Murdoch said in a letter to Bradley, dated July 14 and published on Wednesday.

“There is also the broader risk of a potential harmful effect on other companies’ inward investment decisions currently under consideration in the UK.”

Fox executive chairman Lachlan and chief executive James said the deal was one of the biggest investments in Britain since the country voted to leave the European Union last year.

“Our proposed transaction will be carefully scrutinized by others keen to gauge the government’s commitment to creating a climate conducive to investment, or in the words of the Prime Minister and several of your fellow ministers, ‘open for business’,” they told Bradley.

Regulator Ofcom has assessed the deal and found it had no concerns about Fox’s genuine commitment to broadcasting standards, a view Bradley said she was minded to endorse.

But it was worried about the level of influence the deal could give the Murdoch family over Britain’s news agenda and political process, and said it should be referred to a lengthy review, which could push a final decision well into 2018.

Fox said it was “disappointed” by Bradley’s intention to refer the deal, although she still has to confirm her decision after receiving further submissions.

The Murdoch family, led by 86-year-old Rupert, has long coveted full control of Sky, despite the damaging failure of a previous attempt in 2011 when their British newspaper business became embroiled in a phone-hacking scandal.

The family’s Fox offered $14.6 billion to buy the 61 percent of Britain’s Sky it does not already own in December, re-igniting a debate about their political influence.

Lachlan and James urged Bradley to make sure political considerations were not allowed to cloud the decision-making process.