* BoE's Vlieghe says wage pressure building gently
* Slack in economy diminishing - Vlieghe
* Vlieghe considered MPC member most supportive of low rates
* Sterling jumps again
* Two- and five-year gilt yields hit post-Brexit high (Adds Barclays comment)
LONDON, Sept 15 (Reuters) - The prospect of British interest rates rising for the first time in 10 years grew on Friday when a Bank of England policymaker who had been its strongest advocate of ultra-low borrowing costs said rates might need to rise in the coming months.
The comments by Gertjan Vlieghe largely echoed the BoE's message a day earlier, when it surprised investors by suggesting it could raise rates as soon as at its next meeting on Nov. 2.
But coming from Vlieghe, they prompted investors to double-down on a rate hike.
Vlieghe was the first BoE rate-setter to vote for a cut after Britain's shock referendum decision in June 2016 to leave the European Union. In July this year, he said a premature hike would be a bigger mistake than one that was slightly late.
After his speech, sterling hit its highest level against the U.S. dollar since the Brexit vote at $1.3616 as investors bet on the BoE following through on its message, despite false starts in recent years after signalling higher rates.
Yields on two- and five-year British government bonds, which are sensitive to short-term speculation about rates, also hit their highest level since the Brexit vote.
Ten-year gilt yields were set for their biggest one-week jump since June 2013, with a rise of 32 basis points.
Vlieghe said he had previously objected to a hike given Britain's slow economy and the lack of inflation pressures other than the temporary impact of the Brexit fall in the pound.
"But the evolution of the data is increasingly suggesting that we are approaching the moment when Bank Rate may need to rise," he said.
"If these data trends of reducing slack, rising pay pressure, strengthening household spending and robust global growth continue, the appropriate time for a rise in Bank Rate might be as early as in the coming months."
Vlieghe said there was still a risk that Brexit will hit the economy harder. "But for now, it seems the net effect of the many underlying forces acting on the UK economy is that slack is continually being eroded and wage pressure is gently building."
Several economists said they did not share the BoE's conviction that the economy will soon be ready for a rate hike.
Analysts at Barclays said they now expected the BoE to raise borrowing costs in November - but not necessarily because of the data.
"Even faced with disappointing data, the MPC might just have boxed itself into a corner," the analysts said.
Allan Monks, at JP Morgan, said he was not sure the BoE really was on the verge of its first rate hike since 2007, especially given the emphatic 7-2 vote by Monetary Policy Committee members this week to keep Bank Rate at 0.25 percent.
He said it was possible that the BoE wanted to prepare financial markets for a hike, after its previous, subtler warnings fell on deaf ears.
"A second explanation for the BoE's policy inaction yesterday is that it still wants to have its cake and eat it i.e. effectively generate some degree of tightening without actually changing rates," he wrote in a note to clients.
Brexit represents a major challenge for the BoE.
The fall in the value of the pound since the referendum has propelled inflation to nearly 3 percent, above its 2 percent target. But economic growth remains weak and there is little sign of a strong pick-up in wages.
Vlieghe said that wage growth was not as weak as earlier in 2017 and that he expected it to pick up further.
Earlier this week, official data showed Britain's unemployment rate fell to a four-decade low of 4.3 percent while inflation rose to 2.9 percent, above the BoE's 2 percent target. (Writing by William Schomberg; Editing by Hugh Lawson)