Garry Monk and his agent have been accused by Middlesbrough of deceiving them in the transfer market to the tune of millions of pounds in an explosive legal letter sent to the pair.

As Sportsmail revealed last week, Boro are investigating a number of transfers that were completed while Monk was manager for less than six months in 2017.

Sportsmail can now reveal the extent of those concerns, which involve potential conflicts of interest in deals involving Monk and his agent James Featherstone, a wiped laptop and unsolicited approaches to players.

Garry Monk and his agent James Featherstone (L) have been accused by Middlesbrough of deceiving them in the transfer market to the tune of millions of pounds

Boro fear Monk's dealings were not protecting the business interests of the club — as his contract demanded he do.

News of Boro's investigation emerged after Birmingham chief executive Xuandong Ren revealed Monk's dismissal as their manager came after Birmingham told him they would not allow Featherstone to be involved in so many transfer deals.

Now, Sportsmail has seen evidence that highlights Boro's concerns. The deals under scrutiny were all completed in the 2017 summer transfer window and involved the signings of Cyrus Christie, Ashley Fletcher, Ryan Shotton and Marvin Johnson.

Boro were unhappy to discover Featherstone, their manager's agent, had been acting for Derby County when they signed Christie from the Rams in July 2017 for £2.25million.

But it was only when they contacted Derby last year, and officials at Pride Park shared information with their Championship rivals in the interests of transparency, that they discovered Featherstone contacted Derby about a possible deal within 24 hours of Monk deciding he wanted to sign the player.

Boro believe Monk had a duty as the manager of their club not to share confidential information about their transfer targets.

Soon after taking charge at Boro in June 2017, Monk discussed the need for a new right back with his employers. Two other players as well as Christie were considered, although sources at Boro claim the new boss expressed certain concerns about Christie.

Boro are investigating a number of transfers that were completed while Monk was manager

But when it became apparent their primary target wanted to stay with his newly-promoted club and a senior Boro official informed Monk their second choice was also proving difficult to lure to the Riverside, Monk said they would have to consider other options.

He said as much in a text exchange on June 28, 2017, and told a member of Boro's recruitment staff he had since heard more positive reports about Christie.

As Derby told Boro in an email in August 2018, Featherstone contacted Derby and said in exchange for a five per cent fee he could secure a deal for Christie to join Boro. Sportsmail understands that Featherstone contacted Derby on June 29, less than 24 hours after Monk had conceded Boro would have to pursue Christie.

On the same day, Featherstone reached his agreement with Derby to receive five per cent of the transfer should it be completed.

Boro had wanted to sign Christie for less than they eventually paid. It seems they valued him at closer to £1.75m.

But Featherstone urged Derby to hold out for a higher price and on July 7 the deal was eventually agreed at £2.25m, with Monk's agent named on the transfer documents that were lodged with the FA. He was paid more than £100,000 for his part in the deal.

News of investigation emerged after Birmingham chief Xuandong Ren (R) revealed Monk's dismissal came after club told him they would not allow Featherstone to be involved in deals

It was only as the deal was about to be completed, however, that Derby informed Boro that Featherstone had acted for them.

Boro claim they were unhappy about that and told both Monk and Featherstone.

By then, senior figures at the Riverside had already made it clear they were not in favour of Featherstone being involved in their transfer business.

It is claimed that, at the time, Featherstone told Boro he was on a retainer at Derby to shift certain players, and they accepted the explanation.

Now, however, they are concerned that, if Monk did tell Featherstone about his interest in Christie, their former manager could have been in breach of contract and fiduciary duty, causing a loss of £500,000 given what the club had intended to pay for the player.

Monk and Featherstone rejected the claims last year and continued to do so on Thursday.

Of further concern, claim Boro, is the fact the hard drive of Monk's company laptop had been wiped after he was dismissed by Boro chairman Steve Gibson.

The deals under scrutiny were all completed in the 2017 summer transfer window and involved the signings of Cyrus Christie (R), Ashley Fletcher (L), Ryan Shotton and Marvin Johnson

In that 2018 letter sent to their lawyers, which it is understood included a proposal to make a claim to the High Court, it is believed other concerns about Monk's brief tenure were raised.

Monk and Featherstone like to work closely together. So close, in fact, that when Featherstone sent a presentation to Birmingham in February 2018 in what proved a successful bid to secure the job for his client, the agent proposed a role for himself as head of the club's recruitment and scouting department.

Boro claim that when Monk signed Ryan Shotton from Birmingham for them, a prominent agent told them Featherstone had asked to be included in the deal.

At the time, Boro recruitment staff had identified a number of options, but Monk insisted on Shotton. Shotton was not represented by Featherstone but it raised questions as to why Monk was so keen on the player.

Shotton started one league game in September 2017 and no more until December. Monk was sacked on December 23.

Club insiders claim that when Boro were signing Danish striker Martin Braithwaite from Toulouse, Featherstone contacted a senior Boro official about the deal and was told his involvement was neither needed nor welcome.

Monk was told that in future he was not to share such information with his agent.

Other signings nevertheless trouble Boro. They paid £2.5m for Marvin Johnson and he started just six league games for Monk.

The club also feel they seriously overpaid for former England youth international Fletcher from West Ham in July 2017.

They believe Monk may have known, from his previous job at Leeds United, that he could have signed him for less than half the £6.5m fee Boro paid.

Boro were told by Elland Road officials that when Monk was at Leeds, the Yorkshire club negotiated a deal in January 2017 to take Fletcher on loan from West Ham, with an option to buy him at £3m.

When at Leeds, however, Monk would not authorise a loan deal for Fletcher and instead signed Swansea's Mo Barrow, a Featherstone client.

Six months later, Monk recommended that Boro sign Fletcher for £7m, despite the fact that in that time the player had made just four substitute appearances — totalling 53 minutes — for West Ham.

In the end, Boro bowed to pressure from their new manager and paid £6.5m.

Garry Monk spent £50million during his tenure on Teesside between June and December 2017

Boro would now argue they suffered a £3.5m loss and they also have evidence that Monk texted West Ham on June 20, 2017, to inform them of Boro's interest in Fletcher, without the knowledge of the club.

At the same time, Fletcher's father claimed Featherstone had approached him suggesting he act as the player's intermediary or be involved in some other capacity. A Boro official sent Featherstone a text asking him not to contact the father again.

Sportsmail understands that Boro believe Monk and Featherstone had conspired to harm the club unlawfully, although as things stand any action has not progressed beyond that initial communication and the response provided by the pair's lawyers at Mills and Reeve.

Their investigation has not been helped, Boro claim, by the deletion of files on Monk's company computer. There does not appear to be a breach of either FIFA or FA regulations. But while Boro's inquiry continues, the latest FA regulations for intermediaries address some of the issues when the agents of managers get involved in club transfers.

In particular, there is a regulation that states 'intermediaries, players, clubs, club officials and managers shall disclose in writing any actual or potential conflict of interest they might have in relation to a transaction and shall obtain the express written consent of the other parties involved in the matter (in the form prescribed by The Association from time to time) in order for their activity in the Transaction to continue'.

Boro fear Monk's dealings were not protecting the business interests of the club — as his contract demanded he do

Featherstone said on Thursday night: 'Allegations were put to Garry and me by (Middlesbrough). We instructed lawyers, cooperated and responded fully.

'We received these allegations almost a year ago and following a strong rebuttal of all allegations, have heard nothing further from the club and there have been no regulatory breaches nor any court action taken.

'The club were always, and still are, fully aware at all times of all transfer dealings. No business was conducted without Middlesbrough's authority.

'As such we remain perplexed by the foundations of this supposed internal investigation.'

There has been acrimony since Monk left Boro. There was a legal dispute between the club and Monk after the 40-year-old was sacked over his contractual entitlement.

In April 2018, it emerged that Boro were suing Monk's backroom team, as well as Birmingham, after they employed the group at St Andrew's.

It is understood the two parties reached a settlement.