The government approved the sale of Cobham to Advent International on Friday, after the deal was delayed because of national security concerns.

Former First Sea Lord Admiral Lord West said Cobham holds defence technologies which are "critically important".

But the PM said "a lot of checks" had been gone through to satisfy concerns.

Speaking on a trip to see British troops in Estonia, Mr Johnson said: "I think it's very important that we should have an open and dynamic market economy.

"A lot of checks have been gone through to make sure that in that particular case all the security issues that might be raised can be satisfied and the UK will continue to be a very, very creative and dynamic contributor to that section of industry and all others."

'Deeply concerning'

Advent International made its initial offer in July and it was approved by shareholders in August.

The government ordered a review from the competition regulator, the Competition and Markets Authority (CMA), a week after Admiral Lord West expressed concerns, in an interview with the Daily Mail,

The CMA's report, published at the end of October, said the MoD had outlined two main areas of security concern over the sale:

The risk of "unauthorised persons" obtaining information about the MoD's capabilities and activity
The threat to existing MoD programmes (due to funding cuts or moving capabilities "off-shore")
In a statement on Friday, Business Secretary Andrea Leadsom said she was satisfied the risks that had been identified had been mitigated "to an acceptable level" - and allowed the deal to go ahead.

But Admiral Lord West said that while he was "delighted" that was the case, "it does mean that there are national security risks that are being mitigated".

He stressed the importance of maintaining defence capabilities in what he called a "chaotic and rapidly changing world where old alliances are no longer certain", adding "no other advanced industrial nation and certainly no permanent member of the UN Security Council is so cavalier about giving up such capabilities".

Sir Ed Davey, acting leader of the Lib Dems, described the move as "deeply concerning".

He added that "we have yet to see evidence" that the previous concerns over national security had been mitigated.

The decision to approve the takeover was described as "deeply disappointing' by Lady Nadine Cobham - part of the family which set up the UK firm.

She criticised the timing of the announcement, saying it was "cynically timed to avoid scrutiny on the weekend before Christmas", adding: "In one of its first major economic decisions, the government is not taking back control so much as handing it away."

'Long-term assurances'

Shonnel Malani, partner at Advent, said the firm took the takeover "seriously".

"We are confident the transaction and undertakings being given on national security, jobs and future investment, provide important long-term assurances for both Cobham's employees and customers, particularly in the UK and also globally," Mr Malani added.

Cobham, which employs 10,000 people, has extensive contracts with the British military and is seen as a world leader in air-to-air refuelling technology.

The firm, based in Wimborne, Dorset, also makes electronic warfare systems and communications for military vehicles.

Its expertise played a significant role in the Falklands War, allowing the Royal Air Force to attack the remote Port Stanley airfield.

'Meticulous' decision

Mrs Leadsom said the decision had been "meticulously thought over" and that she had taken advice from the defence secretary and the deputy national security adviser.

She added that sensitive government information would continue to be protected under the new owner and existing contracts would be honoured.

The company is also obliged to give the government prior notice of any plans to sell the whole, or elements of, Cobham's business.